Provided that the mixture is correctly set (leaned with increased altitude / reduced air density) there is no fuel wastage, in fact the engine becomes more efficient.
However, if opening the throttle further results in no increase in MAP or RPM then there is no more power to be had compared with that at lower altitude. This is due to the lower mass flow of fuel /air mixture through the engine.
In practice, this means that as the aircraft climbs, the throttle would need to be opened wider to maintain the same power output.
When the throttle is fully open, you will get no more power from the engine - that's when you need forced induction.
Rolls Royce invented a two speed supercharger for the Merlin X engine. The low gear speed was used at low levels and the higher speed for higher altitudes.