It all depends... I have the M5 front bumper, but std fenders. I feel that this is a "acceptable" compromise between M5 look but not trying to fool people. M5 badges is a big no no in my opinion.
This is going overboard IMO:
http://www.autogespot.com/bmw-m5-f10-4/2014/09/29#img1
Only thing that gives the game away is the photo of the front brakes...