You can quite easily and successfully run a
sps tank on T5 only. Actually, right now, the big debate is whether or not T5's cause bleaching from too great a PAR jump between
MH to T5. I personally like the shimmer effect of metal halide, but I'm currently trying to decide between switching entirely over to T5 or a mixture. Personally, I say go with a 6 or 8 bulb fixture, you'll be set for most clams (croceas only in the top part of your reef) and just about any
sps, including the high light loving ones like Oregon Tort. The old myth of T5's not being powerful as
MH has been raging in this hobby long enough,
MH is great for what it does, (spotlight effect) but without high end reflectors, no lamp, flourescent or metal halide, is going to be worth a darn tootin. If you go with T5, make sure you get a fixture with good quality individual reflectors. If you go with metal halide, get Lumenarc type reflectors. Reflectors are in some ways, every bit as important as the lamps. Quality T5 bulbs are a must, as is finding the proper mix of colors/spectrums. T5 lamps tend to be very narrow banded in their color spectrum, much more so than
MH lamps which have a broad spectrum with a strong spike at a particular temperature. This can be a PIA until you find the sugar combo, but it can be done. There are some jaw dropping
SPS reefs out there with T5 only, I think you're trackin just fine. And btw, watts per gallon is worthless. You could have 500w
pc lighting and 238w T5 and the T5 would bleach anything that had been under the
pc. It's all about PPFD or PAR (photosynthetically active radiation) the actual light which is used by your corals and clams to make glucose. The watts per gallon rule is archaic and misleading. You can accomplish just as much on a couple hundred watts of T5 as on 500w of
MH. The difference is effect...
MH has the shimmer effect and spotlighting effect which throws things into higher contrast/relief, T5 has a steadier overall coloration but very narrow bands of color. Personal preference and as always, husbandry practices make the difference. Hope this helps.