I'll try and find the thread where I heard it, but the argument was that because of the smaller diameter bulbs, T5 was able to take advantage of parabolic reflectors and the bulb itself didn't stop as much reflected light being blocked. The claim was that without the reflector, T5 was as good as PC but with the reflector, they blew PC out of the water. It is supposedly one of the reasons why DE MH are so much more efficient than SE bulbs - because they can better take advantage of a good reflector.
Just what I heard... at least, I think I did....
- Chad