Unless you flower early and keep them short and not too bushy, I believe you will overcrowd the light.
I have 9 under a 1000w and I should have half of that..like 4 or 5 max.
One think I can think of is that you could add another 600w, and run them on different timers so you only have 2 on at any given time... That would be wicked...
ex.: I-I-O-O, O-I-I-O, O-O-I-I
Good luck