[Vic55]

Well the X5M has a readily breakable ECU for "real" tuning vs the not so perfect tunes on the F10 M5 (resistor tunes ala juice box style). So you are first off getting the full compliment of the boost adjusted with the tune and the tq feel is probably what you are feeling down low. Further, the tune most likely adapted your throttle input so the boost is on with a short press (accelerator) vs the oem press on the M5. So couple that with the tq and hp bump and I can see why you will feel that the car is quicker down low. But the X5M doesn't have a DCT and as said above weighs much more so when once you are in a rolling race the stock M5 will still pull IMHO.

I have the similar feel with my SL63 (tuned to almost 675 wheel tq) vs my stock Porsche TT-S. The SL just feels like it pulls better because of all the immediate tq down low but I know which is the faster car. The Pcar revs higher and its tq peaks around 5k but the hp keeps climbing much past that number so while I know Im faster I don't necessarily feel that way.