ab, I don't know anything directly about the Defiant but it was more a bomber destroyer than a fighter. As a fighter it lack the svelte shape of the single engine fighters of the day and was thus slower and less manoeuvrable.
As for the turret gunner, other aircraft had similar gunners but few would have been quite as agile as the Defiant.
The following article would explain things better.
Boulton Paul Defiant - Wikipedia, the free encyclopedia