I would go with 8 bulbs. You can tinker with your colors a little more. If you decide to go with a retrofit kit get reefgeek reflectors. They don't show them on their site so you have to call them for it. They currently have reflectors that give the highest par.
Correct me If I'm wrong - Whether you have 2 x 150W MH or 8 x 39W TH, aren't they both generating approximately the same amount of heat?
300W MH vs 312W T5 (if you do 7 bulbs then its 273W)