A brushless motor upgrade would require a brushless speed controller upgrade, and I'm unsure of whether or not a 3rd party speed controller would work with the Slo-V's standard receiver module. You could very easily spend twice as much again the original cost of your Slo-V by upgrading the motor, ESC, and receiver/radio system.

My advice is to enjoy the Slow V for what it is, and if you want a brushless version just go ahead and build a GWS Slow Stick with brushless components and a 72Mhz or 2.4Ghz radio system.