@TheFlightGuySP Could work, shall try as free time comes. Thanks.

Anyways I created my own code for the supercharger simulation (funky trees) without touching the prop pitch:
Throttle + Altitude>3000 ? Throttle*1.31 : Throttle as shown on the aircraft being flown.
Altitude>6000 if you want, at high altitudes.

@AnOmalOcaris Right. However, if the throttle input goes beyond the RPM limit, then throttle governor prevents any further power being applied to the engine, regardless of how high the throttle input is.
I have a ducted fan engine that doesn't really increase power after 50% throttle, because it reaches the RPM limit.
You might try having the propeller's throttle input be an activation group, and have the pitch mode set to Manual with throttle governor enabled, and then put Throttle as your propeller pitch input. This way, instead of directly increasing the power by throttle, the engine is always either on or off, and thrust is changed by changing the propeller pitch. Higher propeller pitch works good at higher altitudes where the air is thinner.
I suppose in a way, this changes the engines efficiency manually with the throttle.

@TheFlightGuySP Im not touching the RPM. Just the throttle.
Going beyond the maximum value of 1 by a funk code

The problem with propeller engines is that they are limited by their maximum RPM. Increasing the input alone may not be enough to actually change the performance of the engine.
You will most likely need to change the maxRPM value.
You could bypass the throttle governor, but then you have to deal with potential prop overspeed.