Either way, if you want more speed, you have to get a new controller...
Since 2008, currie has put a limiter on their controllers... Even if you hooked up 36v to the current controller, you would still be limited in speed.

You need to get a 36v controller and new batteries. Like in the video.. 36v and controller is the best you'll do... If you go 48v you'll burn out the motor.