Ryan,
I bought a couple of 10W to go over a 10 gallon QT because I already had one of the old school screw in lighting fixtures. I wasn't very impressed with the amount of light they put out. That said, if they were 20's and/or were over top of a 2.5 they'd probably put of a decent amount of light. They don't have any kind of built in reflector, so making one would definitely up the output.
When setting up her 2.5, Moriah asked Ebay what they used and they have a 2x9W coralife PC fixture over their 2.5. Apparently their light is the only source of heat and they don't need to cool it. The tank seems to do pretty well.
Because I took lessons from Tim Taylor I thought Moriah needed more power, so I recommended she go with a 2x18watt coralife fixture. It definitely provides PLENTY of light so that she can definitely keep any softies or SPS that she wants, but there were a couple of issues with going bigger. The fixture is wider than the tank (but the opening where the bulbs are is not), so we needed to find a way to get the light mounted over the tank. The fixture also puts off quite a bit of heat, so we needed to buy a little computer fan and mount it in the "hood" that we built. This upped the evaporation, but also probably helped a little getting oxygen exchanged at the surface (a potential problem because you can't really have any kind of surface skimmer on something that small). I would eventually like to build a stand so that it balances out the look (it's a small tank, so even a small "hood" looks big) and so that I can put a hinge on the hood (right now you have to lift the hood off everytime you want to get into the tank). Moriah said she'll post pictures of her tank later on to give you an idea of what she has going.