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.
2017 Audi R8 V10 Suzuka Gray
2017 Mercedes Benz C63S Diamond White
2016 AMG GTS Edition 1 Selenite Grey
2016 Porsche Turbo S Carrera White
2012 Mercedes Benz C63 AMG Black Series Obsidian Black