Having tested the Mosquito Mk VI, and the Spitfire Mk VIII, there is either a global bug causing all aircraft to be faster than they should be or a global bug causing air speed to be reported as higher than it is.
I reached 367mph on WEP at 200ft in the Mosquito, up from 338mph in 2.17. I reached 362mph at 200ft in the Spitfire Mk VIII before auguring it intentionally, although it was still accelerating. It also had a top speed at that altitude of 338mph in 2.17.
When I was flying the first night the WWI fighters were out, their speeds did seem a mite bit high to me, but I am not an expert on them so I shrugged it off.