I did the same thing for most of this xpac Delk, kept him at 3039 haste with EM and stacked as much crit as possible. It felt very good, but ended up switching to AS and higher haste breakpoint during Lei Shen progression. I still run the high haste setup, and it is by far the most common you'll hear recommended on here, but I agree a full crit build had a ton of advantages.
Definitely felt a bit slow/sluggish in comparison to my current setup, with the lack of an insta-CD and being so much slower by default, but it was stronger in other areas (mana regen by a mile). Actually having HTT baseline now would make EM even better as well, I used AG a lot in Throne but always having EM available for HTT is very appealing. Have been very pleased with numbers so far in SoO but might go back and test the full crit build at some point.
For what it's worth, these are the numbers that RSS is giving me for the value of each stat through the first 13 bosses in 25N.
Stats required for 1% throughput gain:
INT - 577
Mastery - 1265
Crit - 932
Haste outside breakpoints - 826
Average HP% level - 77.4%
Spirit for 1% regen gain - 250
Crit for 1% regen gain - 737
Total throughput from Mastery - 7.9%
Total throughput from Crit - 13.3%
So, Mastery is closer in value to Crit than it was in T15, but both stats are pretty weak from a throughput/scaling perspective. Int is less than twice the value of Crit, but I am skeptical that gemming Crit will yield greater returns than gemming INT. Overall, our stats scale fairly poorly - needing well over 900 secondary stats for a 1% output gain makes me think we will scale worse than other healers.
That gives me little hope that mastery will actually pull ahead by a significant enough margin on heroic. May be it'll be even with crit there in regards to aoe healing throughput. We should probably just avoid the stat on BiS lists completely, as resurgence is just too good to pass up and crit will still be worlds above mastery for single-target healing.
Yeah, I mean we already know that Mastery only comes out over Crit from a throughput perspective below about 65% HP for AoE heals (which almost all of our healing is now) and we also know that 1 Crit is worth about 0.33 Spirit from a regen perspective, meaning that adding 3000 Crit means we should be able to drop 1000 Spirit into other secondary stats, so the breakpoint where it's worth taking Mastery over Crit is probably in the 55% or lower range. I just can't see health pools being that low for a sustained period, even on heroic progression. Even on Thok, once they drop below 50%, the phase shifts anyway, so you get only a few seconds of healing where Mastery is doing more for you than Crit. The 5.4 changes have increased the value of mastery, which is good, because the mastery we have on gear/from the raid buff is less wasted, but I really can't see any case where you would want to stack it. Crit and INT are clearly better.
The real question about our 5.4 gearing is whether 2 Crit beats 1 INT in terms of throughput. Conventional wisdom says it doesn't, but a pure mathematical analysis seems to indicate that we possibly should be gemming 320 Crit in yellows, 160 Crit/80 INT in reds and 160 Crit/160 SPI in blues. A lot of Mistweavers have started to switch to taking Crit over INT in situations where you are getting Crit at a 2:1 ratio, in part because of its interaction with Mana Tea. With us gaining a similar interaction through Resurgence, I wonder if Crit gemming may be superior.