I overdrive my 90 gallon SPS tank with an IC660, and LOVE IT!!! I can run 4 bulbs at 320W, and guess what, 6x54W= YUP 320W. In my case, I get less heat from 4 then I would from 6 regular driven bulbs, it saves space, and I only use 1 fan at the end blowing across the top of the reflectors. I change bulbs at 12 months, and it costs me the same as when I was replacing halides at 12 months, so no difference there.
I use a total of 5 bulbs, but the fifth is a "true actinic" from Geismann and is very low par, really only there for dawn to dusk, and a little more blue.
If I were going to do it all over again, I would still use a 660 for the main ballast.