Does this somewhat devalue the use of two on use mastery trinkets? The thought's always been you pop one at start of HA and one 15s later.

That was my thought.

Tbh i can see this getting fixed as it creates a situation that would appear counterintuitive to an average player.

In case anyone hasn't already seen it, Theck has updated his sim for the new GC mechanics (12% proc rate on CS/HotR/Avoids changed from 20% on CS/HotR) and included his usual comprehensive and insightful writeup. Its worth checking out here: Patch 5.2 Tankadin Smoothness Simulations | Sacred Duty

One change Theck makes is he's more granular in describing damage spikes and how the different gearing sets deal with them. Here I'm lumping them into "big, "medium", and "small" spikes but he gets into more detail in his writeup.

I'm personally still thinking through the implications. As Theck notes, many more gearing approaches become viable. Staying with a Control-Haste build (hit/exp capped and putting everything else into Haste), which most folks are doing now, is the best at eliminating high damage spikes, has the best ShoR uptime, has the highest DPS, and has the most controllable active mitigation. On the downside, its actually quite bad at preventing medium-sized damage spikes and takes 20% more damage than a pure Avoidance-stacking build. On the one hand, sure, non-spike damage within a reasonable range doesn't really matter for tanks. But a 20% variance is probably starting to test the "reasonable range" side of things.

Edit - I'll do a quick Pros and Cons. I'm bad at formatting so I apologize in advance if this looks horribly ugly.

Strategies

Control-Haste (fully cap Hit and Exp, all else into Haste) - Current default strategy (except against heroic Sha)
Pros: Highest DPS, great ShoR uptime%, Great at eliminating the biggest damage spikes
Cons: Takes a lot of damage, bad against medium and small sized spikes
How it works: Sacrifices everything else to maximize ShoR% uptime and reliability. If you're good and can time your ShoR perfectly, can do well.
Notes: High performance sports car build. Can do tricky things with timing ShoR other builds can't do, but low margin for error and you're going to get pummeled a lot. Theck says this should remain the choice for high-skill tanks but I'm not sure pulling off the timing required is even possible. As a tank, high-risk builds seem, well, risky.

Control-Mastery (full cap Hit and Exp, all else into Mastery) - Alternate current default strategy (use vs Sha)
Pros: Excellent against big and medium spikes, great WoGs, doesn't really have any weaknesses
Cons: Pretty much none. Still pretty good anywhere it isn't great. I guess ShoR uptime is lowish but it is reliable.
How it works: ShoR isn't up as much as other builds, but when it is it'll stop a TON of damage. Also, much higher Block% than other builds which smooths out incoming damage nicely.
Notes: This is the... um, Mercedes build? Great at eliminating a wider range of spikes than Control/Haste and still does everything else well. Probably the easiest build to play. Theck argues this should probably be the new default gearing choice and I can't really argue.

Control-Avoidance (full cap Hit and Exp, rest into Dodge and Parry)
Pros: Pretty good against spikes of all sizes
Cons: Its basically like Control-Mastery, except slightly worse at most things
How it works: Kinda like Control-Mastery except instead of bigger ShoR damage reduction, bigger WoGs and higher block%, you avoid more attacks. Net result is you take very, very slightly less damage than Control-Mastery but you get worse spikes.
Notes: Its not a bad strategy, its just not as good as Control-Mastery. The, um, Volvo strategy (given that Volvo's aren't really any safer than other cars anymore)?

Control-Balance (full cap Hit and Exp, rest into a mix of other stats) - What you'll probably use as you gear up
Pros: Its middle of the road at everything
Cons: Its middle of the road at everything
How it works: You equip upgrades as you get them
Notes: Its really not a bad strategy with the strengths and weaknesses of the three previous ones all mixed together. Gear up this way taking all the upgrades you can find, then gradually optimize when you can. There's a reason there are 10 bazillion Honda Accords on the road.

Pure Haste (throw everything into Haste)
Pros: Wow, that's a lot of haste. Extremely short GCD is probably fun to play.
Cons: While its pretty good at damage done (Hit/Exp are still better dps stats than Haste, but Haste is pretty good), its bad at everything else
How it works: Can crank out a ton of HoPo really quickly with super-short GCD and ability CDs, but lack of Hit/Exp makes HoPo generation, and thus ShoR% uptime, unreliable.
Notes: It just doesn't work very well, don't do it. Highest incoming damage PLUS arguably spikiest incoming damage is not a good combo. The Delorean of gearing strategies: Sexy, but ineffective.

Avoidance (throw everything into Dodge and Parry)
Pros: Great at reducing damage taken (10% less than Control-Mastery), Superb against Small and Medium spikes. Very strong at AoE tanking.
Cons: Not great against Big spikes but not terrible. DPS is terrible (have fun missing 1/4 of your moves plus a slow GCD), ShoR% uptime is poor AND unreliable
How it works: If you dodge a blow, it doesn't do any damage. Stripping the points out of Hit/Exp is enough to turn the corner on Avoidance, making it reliable enough to make up for the bad ShoR% uptime.
Notes: If the dps wasn't so bad, this might give Control-Mastery a run for its money. However, after 6 months of hit/exp capping and stacking haste, it feels weird and frustrating to play. AoE tanking could be amazing, though. Who cares if you miss a lot if you get to cast AS every 3rd GCD? The 18-wheeler of gearing strategies.

Being a healer, even a 20% greater increase on average tank damage will not run any healer OOM in any way. What matters from a healing perspective is continuous/heavy raid damage and spikes on tanks (or other raid members for that matter) which require either cooldowns or "flashing" back into a comfortable zone.
I honestly don't think any gearing strategy will increase the total damage taken of a tank by such a large amount, that it is actually noticeable from a healers PoV. Reducing those spikes (either with a C/Ha or C/Mastery) will probably still prove to be the most manaefficient way of gearing for a prot paladin if he is capable of reducing spikes effectively with the ressources granted.

Absolutely love your summary. If slashdot has taught me anything, it's that everything is better with a car analogy.

I'm torn between C/Ha and C/Ma myself. Mel's argument about timing has a lot of weight, and the DPS contribution of haste is non-trivial. That said, you can also pull the time-shifting trick with C/Ma, you just don't have as much total SotR uptime to work with. Since my simulation is stochastic, it doesn't model that sort of thing, so it's basically conjecture at this point which is better. Does the extra 10% absolute SotR uptime (i.e. 50% uptime rather than 40%) mean you can apply most of that extra 10% to the high-danger periods? Half of it? Or is the vast majority of it just extra coverage during safe times? It's hard to say with any degree of certainty.

My guess is that C/Ha and C/Ma are both going to be more than sufficient for survivability, and it will simply come down to the encounter. A fight like Sha has mechanics that obviously lean towards C/Ma (or even pure Ma, many tanks didn't even hit/exp-cap for it). A fight with a very tight enrage timer and high Vengeance uptime might slant you towards C/Ma. And as you noted in the comment on my blog, an add fight may even slant you towards C/Av (sims forthcoming...soon?).

By now we're used to keeping multiple gear sets around, so the game of rock/paper/scissors we're dealing with here isn't new to experienced tanks. I think the neat part is that, unlike before, there really isn't one "default" gear set that's best most of the time. That means beginner and intermediate tanks, who might have just built up one set of gear and stuck with it all tier, may now have to start thinking about their gear a little more carefully. I don't think that's necessarily a bad thing.

Thanks, Theck.

One thing that jumps out at me is the step forward the Avoidance strategy makes compared to the Control/Avoidance in terms of spike reduction. It was rather surprising to me and I attributed it to finally having enough itemization in Avoidance to hit a critical mass.

This is just sheer speculation on my part, but if my interpretation is correct, I wonder if next tier (Siege of Orgrimmar! Looks like we're getting 3 tiers this expansion) the increase in itemization available will allow Avoidance to take another step forward. Diminishing Returns might start eating Avoidance for breakfast at that point, but I also wouldn't be shocked to see Control-Avoidance establish itself as an actual equal to Control-Mastery (perhaps explaining why they didn't end up having us go with 0/30 on, GC, could make Avoidance too good down the road). Would allow some really exciting gearing options.