Wait. Are we talking about the heavy mastery prepull set?

Any reason for you didnt include the Tier 13 chest? with the chardev you provided it was a 0.71 increase to mastery if you changed the 391 chest witht the tier one. plus the legs have been added
Chardev

Chardev for some reason limits the gear you see. When you have a warrior class selected it limits to only plate which makes sense, but when you dont have any class selected there for some reason are random pieces of gear that are not shown as well but will show up when you select a class. Not entirely sure why but if I missed something, its probably due to that.

Just curious... how do you deal with the massive threat that this "mastery boost" gives You?

I'm still not quite convinced that Creche is stronger in single-target situations than Bone-linked fetish. 3.5-4% overall damage + passive strenght is quite the amount to beat.

About threat. I doubt tanks should really have a lot of issues since their threat got buffed by (200?)% a while ago. And tbh in the very worst case you'll get hit once, which you'll most likely survive.

Edit:Ran the numbers on Landsoul's spreadsheet and keeping the Creche Equipped and not using the mastery tank trinket is better. Not sure if Eye of the Unmaking has the Same ICD since its a stacking buff.

You are delaying the proc by 100 seconds. Equipping a trinket triggers the internal Cooldown of the Proc, which is 100 seconds for this trinket.

Edit: Apparently the ICD for [Creche of the Final Dragon] is even higher. According to the Fury Dps Spreadsheet-Thread it is 115 seconds.

From my experience with Vessel of Acceleration, stacking trinkets does not have ICD's upon equip.

No it does not. There are 3 different kind of trinkets:

1) Trinkets with a "Use/Activation" Effect. e.g. [Ruthless Gladiator's Badge of Victory]
2) Trinkets with a Proc on hit/crit. e.g. [Creche of the Final Dragon]
3) Trinkets with a stacking Buff. e.g. [Eye of Unmaking]

The first category of trinkets has a 30 second activation cooldown on equip.
The second category triggers the ICD on equip.
The third category normaly doesn't have an ICD at all.

To come back to the original question, [Creche of the Final Dragon] has an ICD of 115 seconds. Because of that you would probably loose 33% Uptime of the Buff considering a fight of ~6min length(Ultraxion HM).
However you could still use a trinket of the third category without problems, if you want to exchange it with a trinket of your Mastery Prepull Set.

Is the necessarily true?
I have been testing this by using a /cast [@focus] spellname macro to a target so that I am not interfered by auto attack.
I have, multiple times, gotten a tentacle proc from using bloodthirst. (It could also be deep wounds tick, but even if so, the ability that gave it deep wounds was dealt by a MH ability.)
I am believing that the weapon doesn’t have to be in the main hand to get the tentacle procs from a MH ability.
Also, if this is true, does that mean it has a lesser proc chance…? I have no idea how to test that.

I have used the LFR and now the normal version of this trinket. It's pretty much consistently up every 2 mins. Lines up with Deadly Calm on my end well. (+/- a few seconds at most)