Anyone run high-flow Cats to reduce hydrocarbon exhaust smell?

Well, I am finding out that with the new ethanol fuels, the plug color readings are a very different animal than in the old days. My son's 340 plugs are so clean that I can't see any color in the traditional sense. So I would not count on that.

If you are smelling fuel in the exhaust (unburned fuel) , then it is truly a guarantee that the O2 sensor based AFR will be off. I am not just arguing to argue or prove I am right; it just is the way they work. An accurate conversion from the free O2 measured in the exhaust to an AFR number REQUIRES the assumption that there is 100% combustion of the fuel. If there is not 100% combustion, then the AFR conversion is guaranteed to be in error. This can all be looked up if you don't believe it.

With cold engines and a lot of overlap at idle for a carbed engine (and even smaller overlaps for EFI'd engines), O2 based AFR meters will have errors.