Watts per gallon aren't going to make a difference when comparing PC lights to T5 or MH. It is the PAR value that makes the difference. You can have T5 or MH lights with less wattage that will give you all around better lighting and coral growth that higher wattage PC bulbs, all because of the PAR (Photo-synthetically Active Radiation)
Photosynthetically active radiation - Wikipedia, the free encyclopedia