Personally I think you have to demonstrate the cars ability but without scaring the poor guy so much that he changes his mind and doesnt buy it!!

I also think its the sellers job to show how quick it is but if the buyer test drives it he/she should drive it carefully and sensibly just to see if it drives ok. Once they've paid up they can then drive it how they like. I dont know where you'd stand if someone test drove it and stacked it and weren't insured!!