Exactly what major effect do you expect wing flexing to have? The difference you are suggesting is not remotely subtle so I can only imagine you're thinking the Spitfire's wings flexed to something like a 30 degree angle from true when it was turning.
That is manifestly false.
What do you think?
The silliness of what you are saying can be summed by this: Can you see the difference in wing flexing from 3 Gs to 6 Gs? It is probably visible, but not by much...
The wings are made to take 12-14 Gs without permanent deformation: Assume I say they take 5.5 Gs worth of load in a 3 G turn, and 8.5 Gs of load in a 6 G turn, where do you come up with a 30° flex?
The real issue woud be the extra drag, but then extra drag compared to what? If it's compared to a jet, then that woud explain why a same wingload Vampire easiy out-turns a Spitfire Mk XIV (gaining about 90° per 360, which is huge) despite the fact that the Spitfire doesn't lose thrust at lower speeds, and has the superior extra power to easily out-climb it by a wide margin...
People clinging to their theories point out the Vampire has about a 4% lesser wingoad, leading naturally to a 20-25% gain in turn time I'm sure
Gaston