As its currently stands, you may as well name the game "Aces High Spit XIV" so that logic doesn't hold up.
It could always have low ENY or be perk. Seeing as it entered service well before the 109K or 190D, or La7. it really isnt all the crazy to ask for. TBH, it makes more sense to make this the default setting for the P-51D since the plane entered service right as this boost rating was made standard. But that's not what I'm asking for.
You could even choose to model it as a type or ord, so that it could be disabled through bombing etc. It would be easy to implement relatively speaking, as no new 3d model is required, and the only changes to the existing FM would be max power/thrust and corresponding changes to the supercharger aneroid settings.
Spitfire Mk XIV High? Really?
Tour 175:
Spitfire Mk XIV 1646 kills, 1847 deaths, k/d ratio of 0.89
Now lets look at the P-51D in Tour 175:
P-51D 13733 kills, 10935 deaths, k/d ratio of 1.26
Oh, look, the P-51D has almost ten times the kills, and a better K/D ratio, than does the Spitfire Mk XIV. Want to rethink your claim.
Well, maybe you meant Spitfire Mk XVI, lets look at it in Tour 175:
Spitfire Mk XVI 7056 kills, 6132 deaths, k/d ratio of 1.15
Well, that is certainly more like it, but still barely over 50% of the P-51D's kills, so the claim still falls flat.
Now, it is worth noting that the Spitfires Mk XIV, Mk XVI, as well as the Tempest Mk V, Mosquito Mk VI and P-47M were also using 150 octane to get higher performance than they have in AH (P-47M may be modeled with it, not sure, not really a P-47 guy, but the others are not, not sure Spitfire Mk XVI's ever operated without it) just like your P-51D.