I doubt the rear-engine would have been a liability in combat... more like a reliability advantage. If it was to be used as a dogfighter though, its greatest liability would have been pilot visibility.
As a point of fact however, there were piston engined fighters of WW2 that exceeded the Do-335, which topped out at 474 mph. The
P-51H model had a higher-powered Merlin variant which, after boosting, could reach 487 mph, though it did not see service in combat. The fastest that I am aware of however, was an experimental model of the
P-47, the "J" model, which could hit 505 mph. It had a bulbous propeller spinner with a fan mounted ahead of the engine, similar to the FW-190 early variants. It also had a much narrower engine cowling with special engine boosting to 2800 hp, and had 2 of its machine guns removed. It was intended to shoot down V-1s, but it didn't proceed beyond the prototype.
