So to make sure I'm reading your op post correctly. You noticed a slight vibration of the steering wheel when you purchased it with the stock wheels, then you installed spacers and still the vibration is the same and also the same with ARC wheels?
Did you at anytime change tires? Or are they the original tires from the day you picked up the car?
I have a vibration also but the story is a little different. It's an '08 with only 37K miles. When I got it new it was smooth all the way up to top end when I purchased it in Europe while living there.
I then installed APracing BBK and had to use 12mm spacers to clear the BBK with the OEM 18" wheels. From that day i've had a slight vibration.
Spacers were a temporary fix till I got new wheels/tires. Purchased ARC-8 18x9.5 & 18x10.5. Just prior I purchased new tires but should have waited till I got the wheels. OEM size. 245/40R18 & 265/40R18.
BMW local dealer Road Force balanced the wheels and it was fairly smooth. No spacers installed with the ARC's. So far I've put on about 8,000 miles on the tires and a few weeks ago noticed the vibration returned. BMW road force balanced again for two days and still I have it. vibration at 80+mph and slightly amplified with braking.
Going to have the tech there road force balance again Saturday.
So I wonder if your problem is simular to mine? Hopefully its just tires and nothing more. Let us know. I'll post my findings too.