My experience has been that faster planes are frequently at a major disadvantage in combat unless the pilot is really good at using the speed and avoiding the situations where it works against them.

Basically in one-on-one, speed is nice. In a 6+ plane heat, it's better to be going the same speed as most of the rest of the planes. If you're roaring around at mach3, someone else is busily running around at 80mph gathering up everyone else's streamer while you're still trying to get turned around and back to the furball .