Yes, it's possible. There are quite a few tanks out there using T5 HO lighting to keep SPS corals and even clams. I have a friend that changed over to them and is keep SPSs, but he's only done so recently so I can't comment on any long term success in his case. But, there's been a Tank of the Month on ReefCentral that ran T5s with SPS corals and it was a pretty amazing looking tank.
I'm not an expert on pairing lighting to tank sizes, but if I had to guess I'd say a 4x54w T5 HO set-up would work for a 55g, especially if you wanted to overdrive 2 daylight bulbs with an IceCap ballast.