Well, I run 4x4 trays with a 1000w and I think it works out great. You don't want them right up against the wall, but I think by the done they probably hang over 6" around each side, giving a 5x5 implant. I generally have 15 plants in a tray. How tall are yours when you flower?
Mine is a drip system in a tray with square pots 10" high, and about 5"x5" at the top, filled with hydroton. I've gotten 2.5 oz of dry bud per plant (dry).