Other than the Italian fighters (whose design load spec was amongst the highest of all nations in WWII), 12G Symmetrical would most likely result in some sort of structural damage/failure in pretty much
any WWII fighter (and just about any current fighter as well). Rolling G damage would occur at very much lower values.
Structural G modelling is in IMO poorly modelled in CLOD and was the subject of debate before release.
This is thread drift though
Edit: Basic Spitfire Design load was 10G. Source : Spitfire at War vol I Ch 19 "Stronger, Safer Swifter" by Eric Newton MBE,Ceng,FRAes