One other option is Speedylink. They are a sponsor at this site and can be found here: http://www.digitaloverdrivesystems.com/
I have only read about their product, I have no direct experience. They sell a device which fools the speed sensor in the controller allowing the cart to get up to 19-20'ish although GolfCart's
solution sounds like more fun as you can build it yourself
There used to be a link to a schematic to build your own speedylink on buggiesgonewild.com, but I can no longer find it.