I think that is fine. Just make sure you keep an eye on tank temperature. I used 130W PC on my 20 gallon. Then eventually I had 250W MH on my 20 gallon tank. If it starts to get to warm use a small fan to blow on the surface of the water. This helps keep temperature in check. However, with such a small tank you will have to keep up with topping off cause you will have lots of evaporation especially if you have a fan blowing on it to keep it cool.

Oh and to answer your question: I suppose it could be overkill - It depends on what are gonna stock in the tank? You did not say. If just LR and Fish - yes it is overkill. Softies - maybe?, SPS - probably not.