The reason T5's are more effective is because of their small size, and the individual parabolic reflectors. Since T5's are so skinny, a properly shaped individual reflector can reflect almost all of the light back to the tank. In theory, you could do the same thing with VHO's, but because of the size of the tubes, the reflectors would have to be much larger, which would limit the number of bulbs you could put over the tank.
Hopefully that makes sense. The key is the individual reflectors. If you're going to retro fit the T5's in a canpoy, I'd suggest getting an Icecap SLR retro (reefgeek.com has good prices, and is a great company to deal with). the SLR's are currently considered by many as the best reflector option out right now. I'm not sure how tall your tank is, but 4 bulbs may limit the depth that you can keep SPS & clams.