Watts per Gallon (WPG) is an old rule of thumb. Doesn't really hold true anymore due to the advances in lighting for the hobby. I don't know exact figures but you have to consider lighting intensity. So lets just say A 39w T5 will put out say 4 times the amount of light a 65w PC light will, but using a WPG formula you would need almost 2 T5s to equal 1 PC. Functionally speaking, you would bleach every coral in your tank and probably need sunglasses to look at it.
T5s provide much better intensity and depth penetration then PC and VHO. I personally like T5s better then MH as I can play with the color a little more using different bulbs where as with a MH I am restricted to only 1 spectrum of light at a time.
T5s run cooler, eliminating the need for a chiller or fans (unless oput in a hood then fans would still be my recomendation).
Retro-Kit
Instead of a lighting fixture it is a reflector, ballast, etc that gets attached to a canopy instead of being a stand-alone unit.
Hope that helps.
