When Fitting a spoiler as well as the extra grip, doesn't it help the car go a little faster due the desturbed air breaking the vacuum behind the car. therefore a rack at the back might benefit the performance.

just a guess.... probably wrong, as i'm sure you'll tell me.

Trust me - it doesn't! I used both boot and roof mounting with the same bikes, on the same car. Roof wins (untill of course you hit a bumpy and the sh1tty Ford welding of the pretend gutter on your Fiesta pulls off!)

Now I have a motor with a bolt on rack!