I took a friend who races at the top level, and he was top 10 overall at GoKart racer in Burlingame, for the YEAR.
Some people just have skills.
I beat Berto last time we went.
I think the counterweighting of karts is a bit of a myth. They MIGHT have different chassis, etc., to accomodate bigger sized people. But the biggest variables in these places is temperature (1-2 HP due to heat is a MAJOR factor) and PSI in the tires. So good luck finding the right rig to win with..