The Free French 520s were relegated to training and then replaced by the Aircobra…doesn’t say a whole lot for it.

Isn’t this more to do with serviceability and spares availability? The Free French didn’t have access to D.520 production until 1944 when, I understand, some squadrons began to use examples ‘liberated’ from Vichy and German sources (which had kept the aircraft in production throughout the war, albeit as a fighter trainer in the latter case).

Without the kind of development that the Spitfire and Bf109 were subject to between 1940 and 1945, it’s difficult to say how the D.520 would have held up. It does seem though that the D.520 was on a par with the Spitfire MkI and Bf109E at the time it met the latter in combat.