Originally Posted by gtb75
As much as I don't like the look, I'll probably end up keeping it... I'd guess the M3 front clip generates additional downforce, so I'm guessing the rear lip spoiler is there to keep things equal.
It's to minimize the Caonda effect. It reduces drag and maybe slightly uplift. It's too small to produce downforce.
But, it should have a slight effect on mileage at speed and top speed. Henri Coanda was an Italian aerodynamacist that discovered this. See pages 137 - 138 of the 7/01 Road & Track. There's a diagram of a spoiler exactly like the M3s showing the effect.
See also: http://en.wikipedia.org/wiki/Coanda_Effect