If oxygen %age of atmosphere was raised to i) 25% . ii) 30% . iii) 35%
How much would it affect the power output/fuel efficiency of current engines?

Engines being 1) Petrol Engines . 2) Diesel Engines . 3) Turbine Engines
and what sort of adjustment could these engine use to make better use this oxygen richer atmosphere?

Biosphere and humans are already adapted to this higher oxygen, so don't worry about that