Just watched this video with a MINI JCW GP behind the new M5 on the Ring. HOLY MOLY! I can't believe how well the MINI goes toe-to-toe with the 560HP M5. Unsurprisingly, the M5 pulls away like a rocketship on the straights, but the M5 really struggles to compete on the corners. I know that the M5 weighs a huge amount more, but it's still surprising just how good the MINI JCW is against the M5. Bearing, this is the GP version, which has a few more HP (only 3 more horsepower, actually) and bigger brakes (6 piston vs. 4 piston on the regular JCW), but I'll bet the JCW non-GP would come very close to what a GP version can do as it's only marginally better.
Quite impressive video!
I guess the reality though is that I'll be driving on the road and not a track and that straight line speed (from the 135 in this case) would be much appreciated. However, there's no denying that the MINI JCW can tackle the corners much better than the 135. Then again on the other hand, the JCW is FWD and the 135 is RWD, so you can turn into the corners better with the JCW, but you'll probably have more fun coming out of them with the 135.