Before I purchased my 5, I test drove several 3's. imo, the 3's, being smaller, cannot attain the same ride as the 5's. On the M, everything is geared towards performance and being that it is a smaller car, the ride is very stiff, non-luxury like. The performance package will be similiar, just slower. Both will do everything you ask of them but neither will have the nice, comfortable ride the XKR has in addition to the performance.
So it's not just a matter of raw power. It's also a matter of concept.
It sounds like you should be test driving a 2004 545. 0-60 in about 5.6.
Here's a link to an excellent review.<---
It seems that the 545 may satisfy your power requirements and provide the ride your accustomed to.
I chose the 530 with lots of goodies. While it doesn't accelerate as fast as the 545, it is fast enough for all situations except drag racing. Since I don't race at all, it's definitely a great ride. The sport package and the 18's do make the ride much stiffer but I knew that going in so that's okay. It sticks like glue to the road, has great sounds, is very comfortable and roomy and basically is like driving around in a well appointed living room.
Since I've drive bimmers for over 10 years, I don't think you'll be dissatisfied with any of them. It's nice to be able to pick the one you like more than the one you can afford.
Hope this helps. Just my 2c.
Good luck and good driving.