Spit IIa

[TomcatViP]

It's a rather long reply needed here indeed.

I think I did alrdy give an answer.

To make it short here :
1st we are talking of turn radius as we are dealing with cte speed turn
2nd it's almost certain that at much of the speed range the SPit had an instant turn speed greater than the 109 as here Wing area rules (at comparable wingspan, nose authority etc.. etc... )
3rd at cte speed, the drag generated by the wing in a turn attitude (AoA) and power to weight ratio are the keys. Simple calculation give you a result dependent only of Wing surface and Power to weight ratio as they are based on simpler theory that does not apply to high speed fighter and high G ( high AoA) turns.

In their calculation they are in effect minoring the drag of the Spit wing.