To try to put lights in watts per gallon doesn't really work anymore. If you put 400 watts of pc over your tank, you will not equal the light of 250 Metal Halide. And thinking that 250 watts of pc would be more light or better light than 150watt of T-5, you would be wrong. There has been so many new lighting advances that that "old" saying doesn't mean anything anymore.
If your tank is a 55 gallon, and you want to keep softies, mushrooms, star polyps, toad stools, your pc will work fine. If you want to keep anemones and harder corals then you need to look at T-5's or Metal Halides. Two 175watt Halides or a T-5 unit with 6 bulbs would work great on your tank.