Pressure drop is roughly 30ft per millibar AMSL. At 3000', pressure drop is 100mb which is substantial. The charge density to the combustion chamber reduces with altitude resulting in power loss. This is offset by the resulting drop in induced drag on the prop. Prop design and diameter would have an effect on the resultant rpm at altitude. All things being equal, there is normally no vast difference in rpms, just significant loss of thrust.
Flew single engine planes and never noticed a significant drop in rpm at altitude once the engine has been leaned out at with rising altitude.