Not really, the floramax is about as good as you are going to get. The thing about fluorescent light fixtures is that their wattage is basically determined by the fixture type/bulb length, so in order to make a substantial upgrade you have to replace the entire unit, not just the bulb.
The good thing with your particular setup though is that the floramax bulb is designed to grow plants, so at least its the right spectrum, just lacks intensity. It'll probably sustain a lot of different plants, but if you get to where you want faster/better growth just look into getting a completely new fixture for it.