+ Faster cast of Holy Light, Divine Light and Holy Radiance:
This might result in increased Holy Power generation if you cast alot of DL or HR. And since 2 Holy light only need 15 000mana (instead of 21 600 of Divine Light) I would assume it to be more mana friendly to cast 2 x 1.25sec Holy light instead of 1 x 2.5sec Divine Light.
+ Faster Divine Plea if glyphed.
Divine Plea reduce your Healing by 50% for 9 sec but give you 30 000 mana over 9sec.
Glyph will make it a castable spell. Increased speed mean you suffer less from the downside of Divine Plea.
+ More procs from RPPM Trinket/Enchant/Gem
RPPM benefit from Haste. If you can increase the proc rate of Horridon Last Gasp by 20% over a 7minute fight, it's alot of mana. Add to that the legendary Meta Gem.
+ More tick from EF (or Sacred Shield)
With alot of Haste I would probably go SS instead of EF. But simply because we would sacrifice alot of Mastery to go high on haste. Also, the lowest mastery we can go raid buff is 19.5%(16.25 when 5.3 is here).
- Less mana friendly
Not sure if realy less mana friendly. Again, if trinket trigger more often and you cast more Holy Light instead of Divine, I would assume it to be the same in the end.
- Less Absorb from IH
Even at 19.5%(16.25% to come), you should still have plenty of Healing from IH.
- You need alot of haste rating, which mean you lose lot of other stats.
If my calcul are good :
1.25sec = 31 000 haste rating.
1.5sec = 18 800 haste rating.
1.75sec = 10 000 haste rating.
Ok, now you may think I am crazy. I agree it is totaly impossible to reach a 1 sec Holy Light. This would requier an insane amount of haste rating. But the 1.75sec is possible right now with 522 gear and you would keep a great level of spirit. The 1.5sec would probably be possible with patch 5.4.
Mastery is geting a nerf for 5.3. Some player are going to crit and other are remaining mastery. I haven't seen much talk of haste.
I don't understand very well RPPM and how it synergies with haste. Would it be accurate to say haste is a mana source if it increase the proc rate of some trinket? And how could we quantify it?