[Murph]

The real P51 (not the one we have in il2) actually performed quite well at all altitudes. The 190A8 was good at lower altitudes, but suffered higher up, which was one of the reasons the 190Ds were developed. I don't know much about the KI84s but I do know they suffered from poor engine reliability, the few that were actually built. Both Germany and Japan by about 1944 had very few really good pilots left, and lots of very green pilots. With a few exceptions, such as the ME 262, they were having to keep modifying airframes that were past their prime. Shortages of fuel and experienced trainers also meant that some new pilots were going into combat with as little as 20 hours in type. One of the reasons Japan resorted to Kamikaze was because even though they could still build airplanes, they could'nt train pilots fast enough to replace their heavy losses.
Conversely, The US was producing large numbers of highly trained pilots by this time, with many of them starting their tours of duty with 200 or more hours in type.
German engineering was very good, but they used slave labor to build a lot of their aircraft, and thus had issues with sabotage and not so great production standards, especially near the war's end. Putting a higher priority on exterminating their work force than on building equipment did'nt turn out so well for them.
There is a whole mythology about German aircraft, but the fact is, they had some truly excellent designs for their time, many of which became outdated by the war's end, and some really awful ones as well.