Meredith effect on the Mustang.
Only the RAE bought into the thrust production theory. Both the NACA and the RLM disagreed.
Understand that of the three, it was the RAE that was trailing in aerodynamics. The British engines were good, probably the best of all the combatants but their aerodynamic sciences was behind the other major combatants. That is why you have RAE claims for things like Mach .98 dives out of the Spitfire that later get retracted as they discovered the static port placement was completely wrong for any degree of accurate speed measurement in the transonic realm.