Everywhere I go I see R56/ R56s with dirty exhaust tips. I wipe mine every 2-3 days then I changed my spark plugs and a miracle happened. I think I changed my spark plugs almost a month ago already and I have not seen my exhaust tips go black like ZERO! I have not changed my driving style and the R56 clocks 600kms a week. So maybe check your plugs and see if they are already eroded and swap them out. The dirt on the tips might be an indicator that you plugs need replacement.

 

And here I was ready to recommend a really good cleaner. Just in case things go south, Barkeeper's Friend Cleanser is great. It doesn't scratch, I even use it on the glass stove top.

 

Barkkeepers friend, use it with some water of course and a clean towel, i would wash the tips wit soap first to get the grim off and so you dont catch a small rock.