"Your lights will go out. The darkness will envelop you. And you will fear the shadows that move within it."
I've actually tried a full crit build in a heroic 25 raiding environment during farm and it wasn't completely terrible. I re-gem'd full crit matching socket bonuses, reforged to the 25% haste plat(raid buffed) extra EF tick, and then reforged whatever left over to more crit. It was a pretty smooth rotation with the extra haste(radiance > radiance > shock(2xDaybreak) > EF) and the extra infusion procs were noticeable. You get the 2xHeal on crit which in effect doubles the absorb amount. The LARGEST downside with a crit build is RNG. It's the only stat we have that is not static throughput. With crit your healing style becomes a lot more reactionary and with crit inherently being RNG it's not good logic. With that being said a full mastery build still come out on top by a pretty decent margin.
If I were to stray away from a full mastery build I would stat weight myself like this: INT > spirit > 25% raid haste plat(not in 5.4 due to hot tick not stacking mastery) > mastery > crit > haste. I would go for gear with the highest INT, spirit, and try to go for secondary mastery. I would gem heavy INT matching socket bonuses: red=int, blue=int/spirit, and yellow=int/mastery. I have a feeling we aren't done seeing Hpally changes for 5.4 yet, so we'll just have to wait and see.