So a little bit more in the way of rudimentary conclusions.
tldr; Incinerate damage you see ingame is inconsistent with this formula we see on Wowhead: Deals 1563 to 1727 (+ 154% of Spell power) Fire damage to an enemy. And there's a correction made for this inconsistency which makes Sac FNB do 2.25% damage less than No Sac FNB.
Start with this: Incinerate: Deals 1563 to 1727 (+ 154% of Spell power) Fire damage to an enemy.
This is the the formula for Incinerate on Wowhead. To make things easy, call the average value 100 (just the formula with 154% of your sp).
Incinerate gets buffed directly by mastery, so the incinerate should hit for 100*(1+X), X being your mastery buffing incinerate.
However, if you go do the calculations on your character right now (WITHOUT SAC) you'll see that your incinerate tooltip and actual damage is actually 100*(1+X)*1.15. There's an extra 15% there when you're not sacrifice spec. I'd say maybe they buffed incinerates formula/spell power coefficient, but I don't think that's an adequate explanation because this 15% is on top of even the mastery increase, whereas a change to formula/sp coef would not yield a flat 15% increase at all spellpower levels.
When you buff sac, you get the extra 15%, so your tooltip and damage with incinerate look like this: 100*(1+X)*1.15*1.15. You can verify this yourself.
Now, FNB we expect to just do the tooltip's damage decreased to the FNB coefficient (just the % on the FNB tooltip), which we'll just call FNB in formulas.
From my tests, it looks as if FNB Incinerate while buffed with Sac are doing this damage: 100*(1+X)*1.15*FNB*.85. This .85 I think is to correct for this 15% increase to normal incinerate. Possibly making the 15% intentional and completely unannounced as far as I know. If you want to test this yourself cast lots of Sac FNB Incinerates, and compare the average value with 100*(1+X)*1.15*FNB, which is just the tooltip you see for incinerate right now times the FNB coefficient. So because (1.15*.85) = .97750, Sac FNB is getting hit by a 2.25% penalty because of this .85 correction factor for the 15% incinerate buff.
Which would make this essentially correct:
Originally Posted by
Zumzumzum
Whaaat
Maybe FnB throws real Incinerates affected by GoSac, so they put an additional modifier on FnB to offset the bonus from GoSac when you have it, and they made a mistake or forgot to modify it after GoSac was nerfed.
I don't think that's intended anyway.
What I'm still confused by, however, is why No-Sac FNB Incinerates are doing more than we should expect (more than 100*(1+X)*1.15*FNB). So while Sac FNB Incinerates are doing 2.25% less than we should expect (100*(1+X)*1.15*FNB), No Sac FNB Incinerates are doing even more than we should expect. Making the difference between the two greater than 2.25%. At this point I think this gap increases as your Mastery Incinerate value (X) increases because this difference is extremely difficult to notice when you have no gear on (only 9% base mastery for incinerate) but becomes much more noticeable when you have lots of gear on (~30% mastery for incinerate).