Aircraft Performance

[ZaltysZ]

Start altitude is the same (good)
Start speed is different (bad),
Stall (the end speed) isn't defined precisely (bad).

Even if you had tested by choosing the same pull up speed for every aircraft (lets say 250mph) and ended test at at 60mph, you still would have pretty insignificant results, because gained altitude would be almost the same (with minor difference). It would be so because of wide interval of speed. Aircrafts like FW190, P47, P51 would have edge in climbing while speed is very high and aircrafts like BF109 and Spitfire would have advantage in climbing at low speed. Your test had included low and high speed climbing, so, for example, what advantage P51 got at high speed, it lost at low speed; for Spitfire it was opposite - it lost at high speed, but caught at low speed. If you had partitioned altitude every 300 feet and had noted the time every aircraft reached every altitude mark, you would see that P51 gained half of altitude more quicker than other aircrafts and then lagged so badly that other managed to caught it.

The bottom line is that you need to fix start and end speeds and then plot 2 charts: "height vs time" and "height vs speed". From them you will be able to conclude at which speeds zoom should be initiated and terminated to again and advantage from zooming. Just by measuring gained altitude at wide speed interval you may get false feeling that aircrafts are balanced.