I've been using the 1 x 55w bright kit to which Andrew referred for several years, and I have not had difficulty growing any plants (as long as other factors such as CO2 and fertilizers are addressed). I have had success with a variety of foreground plants such as glosso, HC, various hair grasses, riccia, etc.
The only drawback to using a single PC bulb over a 20 high is that it doesn't provide even lighting from front to back. By bending the reflector that comes with the 1 x 55 kit in a manner that "opens" it a bit, better spread of light is achieved. The only time I have noticed it being an issue is when plants are grown very tall in the back. The upper fourth of the plants in the back appear to be shaded. With that said, I prefer the ease of maintenance that 2.5wpg affords compared to doubling that with a second 55w bulb.