A gas engine is basically an air pump. Higher elevations get worse fuel economy with the current concentrations. With higher concentrations they could pump less volume for the same amount of oxygen. Everything would need to be re-engineered right down to the fuel formulas to work well with double oxygen, but it would improve things a bit.
Thanks for the lesson, Mr Wizard.
You're confusing 'oxygen concentration' with 'air pressure.'
Oxygen does not burn on it's own. You need fuel, i.e. gasoline or diesel. If you doubled the amount of oxygen present in the air ingested by an internal combustion engine, you would get a horrendous lean condition that would likely result in it's destruction. If it would even run long enough for that to happen.
High elevations decrease volumetric efficiency because the air is less dense. That correlates in no way to 'doubling the amount of oxygen on the planet' or whatever dumbass pseudo-science that hipster retard came up with.