Reason I haven't been partaking in the last couple pages of discussion or updating the OP with it was due to illness, so I apologize. Of course, with the latest changes, all that discussion is more or less outdated and irrelevant.

The 4p t9 nerf was inevitable, and I doubt anyone can say they're shocked or that it's unjustified. It means 4p t9, although still a must have in correct content, will actually be replaced come Icecrown without the need for an even stronger set bonus.

The UB change isn't a huge deal. With GoIT revamped and CotG/Dirge no longer stacking, it's not like we would be able to go out of our way to really max DCs. More or less a straight up 1-2% dps nerf.

The SS change is, of course, the most interesting. Unfortunately, however, looking at the quick math, I'm not sure it actually makes SS all that much more attractive. A physical strike which deals 50% weapon damage plus an additional amount? Plague Strike does just that, so take your average Plague Strike and multiply it by 1.75 (to factor in the additional damage from diseases), and that should give you a rough picture of the damage the new SS will deal... less than it currently does? From what I've seen of datamining and such, SS is 50% + 400, while PS is 50% + 189, so when you add that in, SS will break even? Be slightly ahead? A big change in mechanics, but nothing much in the way of numbers. Not enough to make Reaping worth it, I'm fairly certain, although Epidemic will certainly be taken (as it would have been already).

Of course, there are still many unknowns. Will the shadow damage portion of SS double dip like past forms of Necrosis did? If so, then it's a definite boost, but it probably won't. Will talents like Outbreak be adjusted? So on.

Any which way, it will obviously make the value of ArP shoot up, probably back to Oblit-Unholy levels (or close). The BiS lists will change a lot. Again.

What spec will win out will be interesting, and I'm somewhat skeptical it will still be that GF using one above (For 2H, anyways).

In the new notes this part is relevant to AoE damage with a lot of mobs (posting since it's Unholy's territory):

Area-of-Effect Damage Caps: We’ve redesigned the way area damage is capped when hitting many targets. Instead of a hard cap on total damage done, the game now caps the total damage done at a value equal to the damage the spell would do if it hit 10 targets. In other words, if a spell does 1000 damage to each target, it would hit up to 10 targets for 1000 each, but with more than 10 targets, each target would take 1000 damage divided by the number of targets. 20 targets would be hit for 500 damage each in that example.

Not sure it applies to pestilenced mobs (from the description I'd say no since diseases spread with pestilence are not considered "AoE" as far as I know).

For the new SS, I'm not sure how equal is actually is to the current SS.

Atm, SS does 40% + 10% per disease + 317 all as shadow damage.
3.3 SS does 50% physical + 25% per disease as shadow.

Assuming 6k ap and average base weapon damage of 850, you're white swing is ~2200, and that the disease addition works like necrosis, then SS really does 125% weapon damage.

Current SS => 2200 * 0.4 *1.3 + 317 = 1461 damage
New SS => 1800*1.25 = 1788 damage (assuming boss armor reduces damage by 35%)

1788/1461 = 1.223 = 22.3% damage increase

I excluded some buffs and talents on this, but adding the same multiplier to both means nothing in the end. So the new SS without gaining extra damage on the disease portion, you still get a very sizable damage increase, on top of increased value of armor pen as previously mentioned.

Your math is faulty:
1) The 10% per disease modifier comes at the very end (meaning it affects the flat 317, which you don't have it doing).
2) The new SS does have a flat amount of damage added to the physical portion (and, in turn, this affects the shadow portion, of course). This favors your conclusion, don't get me wrong, but the percent damage increase is totally inaccurate as such, even if it's inaccurate because it's too low.
3) Your new SS math makes no sense. 1800 x 1.25 = 1788? Basic math says no. Boss Armor reducing it by 35% would simply make that 1800 x 1.25 x 0.65, which still doesn't equal 1788. Not sure how you came up with that last number.
So on.

Take your 2200 as your base weapon value. Say that the flat damage portion of the new SS is still 317.
((2200 x 0.40) + 317) x (1.30) = 1556.1
(((2200 x 0.50) + 317) x 0.65) x 1.75 = 1611.8375

Those were the numbers you should have ended up with.

Not that those numbers are even close to realistic/proof of anything/etc, but they're more accurate than what you had.

You're absolutely right about the 317 shadow damage being multiplied too, so the 1556.1 damage from current SS is accurate. As the the 1800, I missed that and should have been 2200 * 0.65 * 1.25 = 1788.

Anyway, I somehow misread of 3 different sources that it does 50% weapon damage + 25% weapon damage as shadow per disease... instead of 25% of the physical damage done. Going to go get glasses now >.>

So then the new SS would only be a significant buff by stacking armor pen. It will be interesting to find out if SS spam will work again or not. Seeing my numbers corrected, I would guess not, and the only other nerf is halving UB damage. Looks like the current rotation may still win for top dps =/

Maybe I'm missing something but is there any point to the bonus damage per disease in the new SS being shadow as opposed to physical or anything else? Unless it differs from necrosis and does double dip on buffs / debuffs then it just seems rather random.

The other factor to keep in mind is that currently all of SS damage is improved by Black Ice (10%) and Ebon Plague (13%). With this change only ~43% of its damage will be affected by these talents, assuming three diseases. Of course that is also assuming that these talents still affect it at all.

I read that as SS damage = (50% weapon damage + flat physical damage) (as physical)+ (# of diseases) x25%x (50% weapon damage + flat physical damage)(as shadow)

or 50% weapon physical damage + 75% shadow weapon damage + some static damage = 125% weapon damage + flat damage

assuming you have three diseases on the target.

It says "25% of the physical damage done" not 25% of weapon damage. That to me means it's based directly off how much damage the target received.

Unfortunately, Scourge Strike seems bugged on the latest PTR Build. It's hitting for physical damage now (only slightly more than Blood Strike) but the additional shadow damage doesn't seem to be there.

faulty math.