T5 are by no means low light, they put out an emense amount of PAR and Lumes per bulb. I have see a 120g tank lit by an 8 bulb t5 fixture, with only a skimmer and calcium reactor and all the SPS had great color and outstanding growth. And the tank was all SPS dominated.
T5 are second best to MH IMO, if not just as good, they put out little to no heat, the light penetrates as deep as MH's, they put out almost as much Par as MH's, and dont have to be changed as often.
Compared to VHOs or PC light, T5's are the best fluorescent lights for SW.