I'd bet P-51D pilots who could catch a Bf 109 at 20,000 feet would be very surprised to learn the Bf 109 actually faster than they were ...
Let's see, the DB 605 made about 1600 PS (1578 hp) on B4 fuel, with MW50 at 6000 m, and 1,160 PS continuous power. 1,850 PS at takeoff at 1.8 ata. If MW50 was not available with the B4 fuel, the boost was limited to 1.45 ata, and it made considerably less than 1578 hp. On C3 fuel, it could make 2000 PS at 1.98 ata when using MW50. There was not a lot of C3 near after summer 1944 (well, at the airfields, anyway ... especially the forward ones), and MW50 was a very short-duration thing. So, it was basically a 1600 PS engine for short duration.
So, the Bf 109K had about 3/4 of the power loading of the P-51D at sea level (in pounds per hp), yet we see Ivanov expects the Bf 109K to climb at 2.8 times the rate of the P-51D at 20,000 feet? Never mind that the BD 605 was a single-stage supercharger and the P-51D had a 2-stage supercharger. I have also never seen a P-51D that had a climb rate of only 2,441 feet per minute unless it was being cruise-climbed at reduced power. Last time I rode in one, we climbed at that rate on GREATLY reduced power, not max continuous or WEP. And we were turning significantly less than 3,000 rpm while doing so.
It may seem like I am trashing the DB 605. Not so. It was and IS a good engine, assuming short TBO is not an issue. But it wasn't going to last very long on MW50, and would be back to startingly normal power ratings in a short time. It would be useful to compare the expected performance numbers at military power only, not assuming absolute maximum power output that could not be maintained for long. When I quote P-51D performance numbers, it is not usually at WER power. A climb rate you can hold for only 5 minutes is not very representative of an aircraft's expected performance in combat.
I notice all the German planes are calculated by Ivanov at or above their often-quoted speeds, while all the Allied aircraft don't seem to fare as well versus the often-quoted numbers. Either we have great PR departments or our engineers are somehow a bit off in their numbers. Perhaps he is calculating the performance of P-51D aircraft in Soviet service using Soviet-supplied fuel and Soviet maintenance? That might explain some of the discrepancies. I can't tell from the post and have no knowledge of Ivanov's qualifications to calculated his grocery bill, much less aircraft performance numbers.
I'll go out on a limb and assume he knows a thing or two about it. But, if so, why are his numbers so much lower for Allied aircraft than our own performance tests show? It would be interesting to find out a few things about his assumptions and methodologies before additional comment.