That may have something to do with it.
100% pitch does not give best speed. It's good for acceleration, but full prop pitch won't make you go faster. In fact in a dive its a wonderful air brake.
Well, atleast up to 4.09b (which I use and have the classfiles) this statement is somewhat wrong.
The IL-2 engine is doing the following (based on very real world behaviour) with the Prop type of the P-51 (PropAnglerType 1) which is the standard "constant" speed type.
If you manipulate the Prop-setting you actually choose a RPM ranging from PropAnglerMinParam (1500 RPM) to PropAnglerMaxParam(3000 RPM).
Let's say you chose 3000 (100 % Prop) and 100% Throttle and then go to 110% Throttle or WEP:
1. Engine generates more power
2. At the current aerodynamic Prop AOA this would create more revs
3. Now the Prop governor increases Prop AOA so it stays at 3000 RPM
(In case PropPhiMax is exceeded no more correction is possible -> Revs will go up)
Now this increase in AOA will generate more thrust -> aircraft will accelerate until sum of drag is equal to generated thrust.
Around 95% of all prop driven aircraft in IL-2 use basically this constant speed type of prop. If you set it to "Auto" the Prop is linked to the throttle setting giving different RPM settings for Throttle settings.
If it really happens that an AC decelerates at full throttle when upping the Prop to 100% something is broken big time in the respective game version (either FMD(buttons) or Code).
The magic in IL-2 is that in using prop tuning one can have the equivalent of +/-1000 HP:
1. Very high Prop-RPM (high Reductor settings, I have seen > 1.0 settings !!!!) -> high Propblade velocity which the game calcs as: square root of(((0.375 x Propdiameter x Pi x rev/s x propreductor)squared) x (airflowV)squared) up to 300 m/s brings only benefits and only small penalty for "supersonic" Props above 300 m/s.
2. PropAnglerSpeed is actually degrees per frame which for 4.09 means degrees per 0.03 seconds (later game versions use > 0.03 s per frame). So the 3°/s of the VDM Prop for instance are 0.09 IL-2 PropAnglerSpeed. If you up this value to 0.15 or higher you will experience dramatic performance improvements of the aircraft.
Together with reductor settings you can easily create a total dog with a 2000 HP engine or an UFO.
Now one cannot really take over real world data into IL-2 unseen (eg. Propreductor settings) because the IL-2 engine abstracts the prop functioning to much for that. So one can only finetune for correct hard fact values as climbrate, level speed but level-, dive- and climb- acceleration can only be guessed. Although for level acceleration the distance needed to takeoff is a very good indicator, because allmost all Aircraft Handbooks give values for that.