I think either the AC15 or the AC20 is going to be PLENTY for a go-kart. People are using them in much heavier vehicles (like motorcycles and threewheelers) with great results. Not only did the Norton team use an AC20, but the Volt (Thunderstruck motors team), Native (electric motorsports team) and the Squarewaveracing all used an AC15 or AC20 on their bike with great results. I think the teams had more problems with getting enough power from the batteries than anything else. The thermal problems were to do quite a bit with the controller not being properly mounted to a heatsink/propperly cooled and going into thermal cutback.
Once the solved the thermal problem, the only issue that remained was lack of power from the batteries. I know the Norton team had Kokam batteries which had plenty of power, and ended up with a podium finish at least once.
They've got a lot of torque/HP for the size, but I'm with Major, I've always ignored those torque curves. I think some of the values are mixed between the controller and motor. It really needs to be properly Dyno'd.