Two things;
1 - If you are running so rich that you have unburnt fuel, as opposed to unburnt oil, then you're not "a little" rich, you're way rich. Being too rich is as bad for one of these engines as too lean. If you wanna run rich, fine, but raw fuel in the exhaust means you are more than a little lean.
And, yes, allowing raw fuel to sit in the engine can be bad for the engine.

2 - If you add length to the exhaust you will alter the back pressure in the pipe and, at a minimum, alter the power band and could seriously affect the power the engine produces.