It would depend on how strong your battery is, and how heat resistant the rest of the light is. /ubbthreads/images/graemlins/evilgrin07.gif
A bigger bulb will in some cases glow only a little brighter and a bit more amber if the battery was only designed for the 75 watt.(not enough current capability).
A High performance grade automotive h3 55 however may actually appear whiter and almost as bright, but run a bit longer on the battery. /ubbthreads/images/graemlins/thumbsup.gif
It's up to you if you wanna give either a shot.