Why should it matter if his car is faster than yours or not? If you enjoy your car and have fun with it then that's all that matters.
Sounds to me like he's yanking your chain and he's winning.
OP I know it is tempting, BMW owners are some of the most pretentious folks out there, and I've had my share of BMW kills with ol' blackie (my GT), but there is no need to get dragged into a race that could have adverse results.
What you should do is rub it back in his face once the maintenance issues start up for that M3, it's going to be spending a lot of time at the dealership
2011 V6 Mustang - Premium 202A Spec (Pony Package) - SOLD!
2014 V8 Mustang - Premium 400A Spec
6 Speed Manual, 3.31 Rear
Black, Charcoal Black
Comfort Package, Security System