There is nothing wrong with the 452mph speed for the Bf109K-4. Before the Bf109s were updated to AH2 standards, the Bf109G-10 did 452mph, which is faster than the Bf109G-10 should be. Pyro, if I recall correctly, stated that they had made a Bf109K-4 and just called it a Bf109G-10 so they could give it the 20mm engine gun option. In AH2 they are being much more rigorous in making the aircraft accurate. See also the changes to the Spitfire Mk IX from its AH1 model to its AH2 model.
The problem with the Bf109G-10 as a gap filler is that it doesn't enter service until November of 1944, too late to be used in settings where the Germans need a Bf109 that performs better at altitude. It can't really be used as a stand in because its performance is significantly better than the Bf109G-6/AS it would be standing it for. It also doesn't solve anything because by the time it enters service the Bf109K-4, with higher performance and good altitude performance, is already available.