If I was you (thank god I'm not) I would use a couple of ATI's positioned where your corals will be and the rest of the lighting made up of cheaper tubes.
I asked a similar question quite a while ago about the penetration depth capabilities of T5's, there was a good discussion on it but no real solid answer came about.
In all honesty, 20 tubes is going to cost a whack, why not try a couple of MH's? Initial start up cost will be heavy, but then it's merely replacing the bulbs every 1-2 years, and for your purpose you can use Venture globes which you can get for a couple of hundred bucks each. A lot cheaper to replace 3 or 4 MH globes than 20 T5 tubes.