If you scroll down you will see that I asked 2 questions about fitting 16" rims to the M6 with factory 18" rims.
Nobody seemed to know, and even my dealer told me they wouldn't fit because the M6 with 18" rims has bigger brakes.
So winter tire choices became very limited.
I asked at my favourite tire shop and was told 16" rims would fit, and Tire-Rack even used 16" as the prefered winter tire package.
I got in the plain steel rims and they fit perfectly.
I looked up the brake parts on internet parts suppliers and for the 2010 6 they were all the same part numbers.
The "bigger brakes" story is wrong.
And for more info, if you use 205/65R16 winter tires, they are very close in diameter to the original 235/45-18.
26.5" diameter vs. 26.3" for the 18" tires.