How Nature’s Grace works with Haste, according to Randomsmo of Chromaggus:
The formula for haste is (cast time) / (1 + haste). So (3) / (1 + .2) = 2.5 is the new formula for Starfire NG procs, NOT 3*.8 which would be 2.4. How this interacts with other haste buffs gets more complicated.
Based off my testing on the PTR for Resto druids:
Nature’s grace is only giving me about .2 seconds off my cast times on nourish as tree spec. With 856 haste (on trinket proc) AND nature’s grace, my nourish spell’s tooltip dropped to .991 seconds. Nourish had a 0.976 sec tooltip with 919 haste (on trinket proc) and nature’s grace.
I’m not convinced that Nature’s Grace will be useful for tree druids in 3.1, as the extra haste bonus will be less useful if we’re not chain-casting Nourish all the time, the way that moonkin would chain-cast wrath & starfire. In addition, faster casting just means that you can spam yourself OOM faster. Those three talents may be better off spent by tree druids in some other resto talent, though I’m sure some healers will still pick it up.
Testing as balance/moonkin spec:
At 504 haste, with just nature’s grace, I’m still not hitting the 1 second GCD, I’m at a 1.05 second wrath cast time (1.26 base cast), outside of moonkin form, so I’m only gaining .21 seconds for a single cast of wrath. If I get off 2 or 3 wraths during the nature’s grace proc, then I at least break even.
In moonkin form, I’m at a 1.23 base cast time with 504 haste, and only getting down to a 1.02 second cast time.
My wrath has a 0.989 cast time with 628 haste in moonkin form, so it has to be about 550 or somewhere around that where you are breaking the 1 second GCD with haste in moonkin form, without raid buffs.
This means that, in general, it’s going to be pretty hard to clip the GCD with wrath if you don’t have heroism, shaman totem, and/or a ton of haste bonuses and such. We’re not GCD clipping on the PTR outside of raids because most people’s wrath casts are NOT dropping below 1 second in normal situations.
In general, the change is a buff for moonkin PvE, as GCD clipping wrath made wrath rotations harder to manage, and if there is clipping, it won’t be as severe (and now, it’ll buff up to 3 wraths if you are casting standing still). This, however, makes the talent more questionable for PvP when you may be moving around a lot (though the haste bonus would effect instant-cast spells).
For raiding moonkin, you are still going to clip the GCD with wrath: For raids where you gain shaman’s wrath of air totem, and full moonkin haste talents, you’ll hit the 1 second GCD with about 400 haste from gear, according to Murmurs.
So, yay for moonkin PvE buffs, I suppose – but the new Nature’s grace won’t actually fix the real problem of wrath GCD clipping in high-end raid situations with full haste buffs.