I've used the T6 Aqua Flora Plus (yes, by Quantum). Its suppose to give you similar to T5 efficiency while going into a standard fluorescent fixture (shoplight, T8, T10, T12). The slimmer profile will let some more light pass with less restrike but I think the biggest difference is the actual output versus power draw due to the smaller bulb diameter. I dont have any actual data though, sorry. But one thing I have wondered is you could probably add an optimal reflector like an IceCap SLR for T5's since its only 1/8" wider the clips should probably just bend a little more. Perhaps I should try that, give me a few days...
And I even considered doing this for my big tank, but to be honest it wasn't worth it. The bulbs weren't any cheaper then T5HO, I'd be using the same reflectors for either, so the only difference in cost is the ballast (which could be little or a lot depending on which you buy). I just switched over to T5HO for the new tank.