Does Unity have a debug menu that shows the framerate and memory usage? You can use this to check relative performance- for example, on my wife's net-book my Python scripts use about 5% total processing time. whilst on my desktop (Quad core 3 ghz CPU plus GTX 460) processing time goes down to about 0.3%.

You may want to decide now what platform you want the game to run on, i.e. optimise the game for low end hardware or go crazy with eyecandy. The main thing is the racing is smooth (constant framerate) otherwise high speed racing will be difficult if you have a 10 frame drop/ increase as you race.