Sikboy, and Sabre, if two pilots of equal ability are in a P51D and the other in a Gloster Gladiator.. I'm pretty positive it would result in a draw.
It's all speculation (based on my experiences, however), but I would wager that a really good guy behind the stick of a Gloster Gladiator would easily hold his own against faster planes.
I know I have no problem dodging someone BnZing me. I never follow someone vertically and save my speed.
So it's just a matter of intelligence and ability of the guy in the slower, more manueverable plane.
Sure, he's got speed and can E-fight me all day.. but if he can't get his guns on target then it's a moot point.
-SW