I just ordered my x6m this weekend, and trading in my 335i. We are expecting our second child early next year so I "needed" a larger vehicle to carry 2 child seats (yeah, right). Prior to ordering, I drove the 5.0 with sport package and it is very nice! With tune, you will be on par with m version. The sport package will be closer to your set up on the tuned 335 than stock, so spring for that if you can.
Agree that the CTS-V is a very nice option but my future stroller hauling days will not allow that. I will beat everyone to the PTA meeting though.