Last year there was a hobbyist race on my local track and the person who won it was a guy who mainly practices at home on his computer. I imagine a gokart simulation is close enough to the real deal that training on it actually can help a lot. You can get much more fine-grained feedback on how well you're taking certain corners etc. One thing that trips me up is understanding the kart - my local ones have somewhat poor acceleration, so it seems it's not actually best to take the corners as sharp as possible and try to accelerate out of the corner, but instead try to maintain as much speed as possible going into the corner, even if you end up on a bad line. This would be easy to test in a simulation, but I just have to go by gut feeling.