I think that fully flapped has missed the point when it comes to aircraft performance. As said above engine performance decreases with increasing altitude but TAS increases.
The effect is that the aircraft flys faster but at the same IAS.
Most engines make 75% power up to about 8000ft so the best place to be for cruise is between 7 & 8000 ft this will give the highest TAS for a lowish fuel burn.
Increaseing the altitude to 12,000 will futher reduce the fuel burn but because at 12,000ft the engine can only make 60% power the IAS & TAS will be lower, range is likely to increase but you now have to look at airframe/engine maintenance costs vs lower fuel burn.
Running at best lean setting should save between 15 & 20 % fuel burn over full rich.