OK, we're all making jokes about propellers on cars, but if any of you are Formula 1 fans you might remember the Brabham BT46B "fan" car from 1978. It was basically Brabham's answer to the wildly successful Lotus ground effects cars, one of which ultimately took Mario Andretti to the World Driving Championships title that same year. Anyway, the Brabham had this huge fan on the back of the car that was designed to suck all the air out from under the car, forcing it to the ground for better traction in the corners. Basically reinventing the famous Chaparral fan cars of the 60s.
Supposedly you could rev the engine at idle and the car would actually squat about 3/4" because of the vacuum created. In it's only race, not only did it run away from the competition, but even with a good deal of oil dropped on the track the car didn't seem to be negatively effected - Niki Lauda trounced the competition and the car was immediately banned from competition.
So maybe we don't need this little prop exhaust for more power, but rather as a means to create enough downforce as to allow us the ability to outhandle Mini Coopers!