I like form and function---- 21 inch wheels weigh alot---- my 20 inch HRE's weigh 21 lbs and you will notice 7 to 8 lbs per corner in the increase unsprung weight. Couple that with increase rotation mass/inertia and performance drops.
But it all depends on your needs and how you drive. I like to drive my cars fast and never want the performance to suffer. I have tested 21 inch wheels on my CLS63TT with HRE and I could clearly tell the difference from a dig and roll with the 21's on vs the 20's and that was with HRE's which are lighter. But thats me. YMMV.
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2012 Audi R8 V-10 Ibis White
2012 Mercedes Benz C63 AMG Black Series Obsidian Black