I talk from my Skyline days when everything wasnt legal. Not going down that track again with the XR6T. Yes lighter coloured cars will stain. Cant remember smell being an issue. As a stock car it made 15 rwkw above an after market cat. Never new of anyone being caught out for cat reasons they were for everything else. Get defected, take bits off, get it off defect put bits back on. People get pretty skilled at turning there cars to stock and back again. Its funny how we can have track cars without cats, but on the road they are required. I have no data to back it up but wonder how well they really work. I have heard after a few 1000 km they are not much chop anymore.