Be careful while *A* T5 bulb might have less power than a MH bulb, MH bulbs can hold their own vs T5s as far as efficiency.
Here's an AA article that has a particular MH bulb, as well as a T5 fixture and a couple of the LED fixtures out there
http://www.advancedaquarist.com/2010/2/aafeature2
In particular 1 250w MH bulb (~300W output) [images are a little too big to post]
http://www.advancedaquarist.com/2010/2/aafeature2_album/figure_1_lumen_max_ushio_250w_14k_de.jpg
vs a 6x39W ATi T5 setup, ~250W output
http://www.advancedaquarist.com/2010/2/aafeature2_album/figure_2_ati_t5.jpg
It might be hard to tell by the charts, but the T5s peak at about 200, where as the MH are almost 300, that's 50% more light peak light for 20% more power. Plus they both have fairly similar coverages (i.e. the halide isn't all at one small spot. It should also be worth noting that this is a 14000K MH vs 6500K t5s, so the T5s should have that as an added advantage as well.