There is more going on under the hood when pressing the M button than just adjusting the throttle sensitivity.
This quote was taken from the M-Power web site. Discussion about the M5 V8 engine.
http://www.m-power.com/_open/b/varli...d=2723&lang=en
"By selecting the Sport or Sport plus accelerator mode, we can really feel the additional boost to the response. How does that work?
In Sport or Sport plus mode, a suitable VALVETRONIC controller and the waste gates keep the turbocharger in the higher speed range. Normally, the waste gate used to regulate the charging pressure opens so that the exhaust gas flows out with the minimum possible loss. Pressure is built up again only when I hit the accelerator. For an improved response, I leave the waste gate closed until I need it for regulating. The exhaust gas then always passes over the turbine, which then runs at a considerably higher speed. When even more power is demanded, it is immediately available. However, I have a gradually rising counterpressure that causes a slight rise in consumption. This function can therefore be switched on and off. By the way, in the BMW 1 Series M Coupé the same function is activated at the M Button"