I have operated from airports with elevations of 10,000 ft and, just as importantly ambient tepms that raise the DH to 12,500ft. the flight manual actually states that mixture leaning for take off is a consideration at high elevations, but I always work from rich to lean, and in any case the engine manufacturers recommendation is that leaning for take off at high elevations should be followed 'to prevent rough running from overly rich mixture', again more important with some egines more than others.
lycoming injected engines are scheduled rich and will almost certainly require leaning for better power, though i never did experience the 'rough running' at full rich as described. monitior the egt and lean with a good rich side margin, aggressive leaning not required, and you will be sweet, however some TC engines are mandated full rich at high (climb) power settings, regardless of altitude (FTH 12-14,000ft) due to the absolute requirement of fuel for egine cooling within limits, remember that engine cooling capacity of air decreases with altitude even though a TC engine can produce 100% power up to FTH. EGT(TIT) can be well in limits but CHT becoms the critical factor. the top part of the green arc (but below red line) is definately the optimal range to operate in any a regular basis. High CHT will reduce engine life as a matter of course.
Oversquare is very good for your engine generally, better oil consumption, and ring sealing higher BMEP aids efficiency and its quieter. if it was bad there would be no TC or SC engines. a merlin will take 62"MAP in MIL, though the boaties were getting up to 120"MAP (though not for full TBO)
HJD