Short answer: DON'T

Long answer: I understand I'm a little late here, but I just saw your post. I would NOT exceed the rating on the fixture. Watts are a measure of electrical usage. It's figured by amps (the actual current), times volts (electrical "pressure"--so to speak). Amps x volts = watts. That's Ohm's Law. Anyhow...the voltage in your house is relatively stable at 120 volts. If you pull too many amps by adding higher wattage bulbs than what are specified, you increase heat, and the possibility of a short circuit. Most fixtures for incandescents are made as cheaply as possible. Knowing this, it is safe to assume the wire size is minimal. If you overload it, you'll prolly start a fire. Worst case scenario...the fire starts when your not home. You come home and the fire marshal is looking at whats left of "some sort of plants" being grown indoors...hmmmmmm??? Not good.

If you decide to use the fixture, that's prolly better. More light is almost always a good idea.
But don't overload the fixture's rating! B.T.W.--I'd only use them to side-light the lower budsites.