Spit IIa is now so much more inferior to the 109

The thick pink line is from handbook. I do not know what these values represent though. Prudence with data whatever source is always prime.

Even flight tests are not representing the final truth as they only represent one individual plane and not average performance.

As Kur already has presented the specs for the 109 which had a guaranteed performance inside a +/-5% bandwidth which is a lot. This however is of course theoretical tolerance. It may have been that the delivered aircrafts were +/-2% from an average somewhere inside the +/-5% (we do not know where this average was and probably the Luftwaffe did not know either).

This should be always kept in mind. Unfortunately this is a big headache for any flight sim developer.

Personally if we could have all flight data of a good statistical probe for each plane I would like to have statistically scattered performances of planes in the game. But this will never happen as we never will have that data.