What kind of metrics ensures us that Defiant was an advanced fighter for 1940?
The technology of the Defiant was excellent. It's low top speed compared to the Spitfire was caused by the turret and the enlarged aircraft designed to carry it. Bolton and Paul could have made an excellent fighter simply by scaling the defiant down and removing the turret. If there was no Spitfire I would expect the specification F.9/35 for the defiant to be for a plain single seat fighter.