You won't reach max rpm at 7500ft that's why it's only producing 75% power with a full throttle setting provided you have leaned it correctly, the reason for it being the optimum altitude is because you will be getting the best TAS versus power and fuel consumption.
You can lean the mixture at any altitude providing you follow the engine manufactors recommendations, on some types you may even have to lean the engine during taxing and certainly if departing from high altitude airports if you don't lean the engine for take-off you won't develop take-off power for that altitude.
Last edited by Above The Clouds; 24th October 2012 at 18:46.