Ok so, my friend and I got to doing some calculations... We came up with some general points for priests...
Regen is only worth getting until you never run out of mana. Having mana regen you can't use = a loss of throughput you could of been getting in other places basically...
Crit is nice but should never be stacked or focused on because it's completely unreliable and often contributes to over healing so never gear, gem or enchant just for it unless it's your only option over like mp5 or something.
Then comes the Haste Vs Sp part of the discussion. We started doing some generalized theory crafting and surprisingly this was our napkin math on it.
Base flash heal is roughly 2100 every 1.5 seconds. It receives a .8 coefficient from spell power. So, it would take roughly 2700sp in order to double the effectiveness of flash heal. However... Haste is a straight thing, not sure how to say that elegantly, however the GCD is hard capped at 1.0 seconds. So, increasing your haste beyond 1.0 second would be pointless because then you would have to wait for the GCD to finish...
TLDR:
It would require roughly 34% haste in order to decrease the flash heal cast time down to 1.0 every cast. Even if you did this you would only be effectively healing ~50% more assuming you spam healed and never over healed the entire fight. However... with only 2700 sp you have effectively doubled the amount you heal for with your flash heal making casting it even at 1.5 seconds more effective than casting it for half that at 1.0 seconds.
So, my question is what would ever be the point of gemming or enchanting for anything other than SP, seeing as it has no DR, assuming you never ran out of mana during most fights?