Shaft power output of your engine is equal to Torque x 2Pi x RPM. If the advertised max take-off power conditions for your aircraft are power lever and RPM lever fully forward, giving max torque and max RPM, then this is what you should get, provided the atmospheric conditions are as specified in your manual. But any changes in density altitude will change the power output.
So if you have the power lever and RPM levers fuly forward and cannot get max torque and max RPM, then either you really are getting less than max power output, or one or more of your gauges are under reading.
If the problem is one of reduced power being generated by the engine, it would most probably result in reduced torque rather than reduced RPM. On the other hand if your engine is producing advertised power at a lower RPM, then the torque should be greater than advertised.
Genghis is correct about the difficulties of predicting what RPM will give best thrust. But if the power going into the prop is reduced, then the thrust output at any given RPM TAS or atmospheric conditions will also be reduced.
So the short answer to your question is yes performance will be reduced.