I had my m3 mapped by Evole, there good guys up there.
You are right the improvement is all top end, although i think thecar felt a little smoother in the power delivery.
it did help the early morning 'SMG' issues which was worth it alone.
They also remove the limiter.
It is also a live map so you do get to see the car before and after the map.
I'll try and post up the graph of the map