We all know what they say opinions are like, but I'll add mine regardless. I've upgraded the size of the wheels and tires on all of my BMW's, and never experience an issue. When I switched to 21" and lowered on H&R springs on my M5, I noticed absolutely zero increase in harshness, and the car feels a little more planted in corners.
Honestly, I make the change for the destinctive and personal look of the car, not to improve performance or handling. As an example, I've never made changes to my OEM wheels on any of my Ferrari's. I just think the look of a Ferrari is absolutely beautiful right from the get go. A piece of art so to speak.
That being said, I've never noticed a negative impact when I upgrade the size on my Bimmers.
As an example, here's a couple of pictures of my M3 with Advan RS 20's dropped on Dinan springs.
2010 M3 Coupe, 2010 X5 35D
2011 458 Italia, 2013 M5
Last edited by cpiguy; 12-25-2012 at 11:42 AM.