This weekend I replaced one of the tires on my 09 CS with 21 inch OEM GTS Sport Wheels, they were mounted and balanced professionally.
I felt some "feedback" on the road through the steering, especially at higher speed >70mph. The steering is perfectly straight, it does not shake, but i sense there is a slight vibration. Which i'm not sure if it's "road feel" or maybe one of the wheels need balancing? I dont remember if I had this "road feel" when i first installed them, but i do remember that the ride is stiffer and I can feel the road alot more compared to my 18's.

Also i have the fabspeed bypass pipes, maybe the vibration is from the exhaust system? Feedback from people with those 21's are much appreciated.

 

Based on you specifics, it is probably a wheel balance issue. The 21" wheels are significanlty more difficult to properly balance. Less likely is a wheel mounting issue so you could loosen the wheels, recenter and re-tourque all of the lugs. Finally, one of your wheels or tires could be out of round.

I would just find somebody with a Hunter GSP9700 balancer and go from there.
http://www.gsp9700.com/pub/search/findgsp9700.cfm

 

Thanks. I had the 4.6is 20's on my x5 4.4 so i know all about how much of a pain it is to balance larger wheels.

Also I've never had 21's so i was wondering if this vibration is normal. It is not consistent, so maybe it's the road? Though Pretty much the "road feel" was always there above 70 no matter what road surface I drove on.