The part which hasn't been answered is, yes, that reduced back-pressure at altitude gives an increase in power.
This is seen in the reduction in max allowable MAP on the R-3350 at 7,500ft vs MSL. It's a supercharged engine and there may be a different altitude for a turbocharged one, but the effect is there.
It's seen in normally aspirated pistons where the most effective operation is at full-throttle height; reduction in backpressure being part of the equation.
G'day
