shifter karts are faster, but the one i modified for a project only did 60mph. you still needed to lean. you were probably still seeing a great deal of slip.
take a look at the steering on a pro kart - the wheels are deeply angled around the kingpin and dig into the ground.
On a track or flat parking lot you could throw the wheel hard left or right even at max speed with no controllability problem other than the aforementioned slight "washing out" if the surface was slick. It would go into a very nice predictable controlled slide.
I miss it! It was great training for later driving a car. I never had the urge to do anything foolish in an automobile b/c the kart was always far, far more exciting. Driving a car was dull in comparison.