Our R56 had done that many miles and performed very well horsepower / trq wise.
It might be worth you booking in for a health check where we would perform a leak down test, we would then put your car on our rolling road, this would then give us a horsepower / torque figure results to compare to other R56 MINI's that we have tested.
Hopefully the results would be line on line and then you have nothing to worry about.
Dyno Rolling Road