A good explanation Silvaire, thanks. It does serve to add to my thought that as combustion is now happening downstream of where it is intended to, although the combustion chamber is not feeling that fire, the open exhaust valve is, and that could be a beginning or aggravating point for sticking the exhaust valves(s) open. The exposed valve stem is potentially receiving deposits of hotter that expected, products of combustion, which had they occurred where they were supposed to, would pass by harmlessly.
I cannot draw a conclusive link between valve sticking, and unconventional mixture settings, but during years of work at the engine shop I saw a number of cases of stuck exhaust valves, and these were often linked to the pilot's report of the use of 100LL, and extreme leaning. I too, lean harder when I occasionally must run 100LL, and stuck two exhaust valves last summer for the fist time in years.
For those not familiar with stuck exhaust valves, the immediate affect is that you are now developing one fewer cylinder worth of power, which on a four cylinder engine is significant.While this is happening, and depending upon which position the stuck valve has taken, worst case engine damage can include broken rockers bosses (expensive cylinder repair or replacement), bent push rods, and cam and follower damage (may fill your engine with metal, and require replacement cam$haft).
To save a bit of fuel, it's just not worth it! If I'm really worried about making the coast of Iceland in higher than forecast winds, it's a chance I might take. For day to day running around, I'd rather waste a gallon an hour....