The M button changes throttle mapping only and does not give you more power. You will get more power on full throttle with overboost with or without M button depressed for 7seconds only!
Changing the throttle map makes the car feel sharper and more responsive. A lot of so called tuners change the throttle map only and then tell the customer that they now have more power.
The only M car I recall that the M button gave more power was in the E60 M5 and E63 M6