The easiest thing to change on the kart for us – is the adjustable rear torsion bar. We run a Birel and changing it from straight up (pushy) to angled (not bad) to out (loose, loose, loose!) is a easy deal for us.
The adjustment that makes the biggest difference though is moving the seat forward and back. An inch either way can make a huge difference. Problem is, moving the seat is usually not a quick and easy thing.