I suggest people talk to someone who owns a C63 with the flat silver paint. AMG has been doing this for the past couple years, in black, silver and pearl white.
I'd be curious to ask one of those owners how the paint holds up before passing judgement. I know the flat black gets dirty very easily, silver may hold up better.
2007 Porsche 911 Turbo, Previously: 1998 M Roadster, 2011 M3, 2005 M3