Sorry I have no experience of this I can give you..
Generally anything is possible, just depends upon how much engineering you want to do....
Personally I would just up the size of the pinion first, if not then buy a faster motor. You could also if your electronics allow go up a cell count battery wise - ie 2S to 3 or 3 to 4S. Or as mentioned above if you can stretch to it get a brushless set up which can offer a larger parameter of speed....
Oh, also, if you are running portals you could overdrive them both for more speed also.... But ideas mentioned already are better...