if i were you, i'd get two 600w lamps. they are the most efficient HID as far as energy consumption goes. not only that but if you have two of them, your lighting should be more efficient around that many plants as well.
bubblebucketing + SCRoG would give you a helluva better yield as well. i bet you could SCRoG out 6 plants 2x2 easily enough under two 600's.
could easily harvest 2 plants everymonth. SCRoG'n it should give you much better yield as if you did have 15 plants. (not exactly but you get the point)
i don't think i could ever see myself throwin out a regular grow without a SCRoG screen. its way more fun training and toying with the plants than just watchin.
i'm no pro at all. but SCRoG with limited lighting and other factors seem to pull off far better yields.
i saw a guy pull 450g off a single SCRoG plant under a 400w lamp.
throw in the rest of your plants and a perpetual grow. you'd be set.