Turbos go a long ways to overcoming the thinner air problem with ICE engines. There is another, arguably bigger reason flying above 10k feet is uncommon—oxygen requirements. Few small planes are pressurized and O2 systems are expensive extras which require additional maintenance and filling. Systems which service passengers as well even more-so.

I flew a Cessna 206U for a few years and it was fine up to 15k feet or more, but I'd almost always skim just below 10,500 so I didn't have to bring along supplemental oxygen. I suspect that's where most of these guys will live, in the 7,000-10,000 foot range. It's much faster then flying near sea level but without the problems with O2.