Even with 'only' 205/55 16" wheels on my non-sp 07 328i, I have never exceeded the adhesion limits of the tires. I am not racing or at any track, but have done some agressive (for me) cornering. When I test drove the sp version, I decided that the tires on 17" wheels had too much 'high frequency noisy' response to small freeway coarseness and grooves. I did not think that the difference in suspension stiffness was that great.
After I bought the 328, for the fun of it, I test drove the Lexus IS 250 with 18" wheels, and the MB 230 with 17" wheels. To my feel the bmw with 16" and no sp is way more precise and planted on the road than either of these. So I concluded that the 16" are good enough for my driving style.
Nevertheless, I still wonder if I would gain a bit of precision by going for 225/45 17's or 225/50 16's all around. I probably would try this if I wanted to go non run flat, but I don't.
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