I did some gear spreadsheeting comparing top level gear in Black Temple between a "Haste" Setup, and a "Holy Power" setup. Couple notes. I loaded up with 22heal gems in all items, except T6 body and Bastion of Light were I have orange/purple gems to fulfill meta gem requirements. Trinket and Metagem regen rates are middle road estimates depending on playstyle. (TLDR @ bottom)
Note: assumes Kings, Lights Grace, 5/5 Divine Intellect, and 5/5 Holy Guidance
Spell Haste Setup: (1.77s Holy Light, 1.36s Flash of Light)
Holy Power Setup: (1.95s Holy Light, 1.48 Flash of Light)
Total Difference-
14stam
2 intellect
32 +heal
2% chance to crit
27 mana/5
So the real question is, how much do you value 2% crit and 27mana/5 compared to the boost in HPS when you can cast 12% more Holy Lights (based on speed) over the course of a fight.
Another chart. I take all spell haste gear and add to it 4pc T5 (head shoulders legs body).
4piece T5: (1.52s Holy Light, 1.36s Flash of Light)

This also loses your 5% Flash bonus, and the 5% HL Crit bonus.
TLDR: Here is the final stats for the 3 maximized gearsets above:
My general opinion. If your a Flash of Light only spammer, and thats fine, going the first route with maxed healing and spell crit, totally ignoring spell haste, is probably your best solution. However, if you wish to maximize HPS, and reduce casting times (keeping Lights Grace up with R4/5 HL and have 8-11 ready at any time), a hybrid heal/haste setup is definately worth a shot, assuming you can get the gear.