What are you really needing to get out of the motors? The 540's with the gearbox are quite powerful and will likley tear anything up you wish to abuse, certainly the torque is plenty to climb anything the tracks can grab! Even vertical!

A purest will use a planetary gearbox to get more torque (and sacrifice speed) even a modest 1:1.25 flyer planetary box in-line with the motor will fit in the gearbox housing with substaintial torque.

You could go for a higher RPM motor in the 540 size that has similar turn ratio just compare manufacturers specs. Or you could get really fancy and go brushless and add extra brush controllers and ESC's, now that would make a hot-rod!