Cant deny I sniggered too
You are right about the old plodder which has an exhaust system which appears to have been designed simply as a dumping mechanism. None of the finer points apply. However, bear in mind that at 10000ft the back pressure is about half what it is at 0ft. My guess is thats got to have some effect but there is a hell of a lot more to it than just atmospheric pressure.
There is no denying that mixture has a major effect on power . For fixed wing aircraft Lycoming recomend leaning at altitude to restore maximum power for a given throttle setting.The implication of that is the richer mixture causes a loss of efficiency.