5-7 watts is just something to go by, a rule of thumb. Other things to consider are par levels, t5 get great lumens/watt. When people mention that you need 5 t5 to compensate for a mh, they are probably talking about an sps dominated tank. For the setup that you want, i dont think you need that many. But joshua was right, you wont the same effect as with a mh, you will loose the shimmering effect. If you are like most of us, you'll slowly want to add more corals.. I have been trying to start a fowlr tank, and it never happens. LOL
Jerry
Trying to get back in the hobby... Will be seting up my 75g rr soon