However, to get back on topic. Looking at the TSAGI book, the performance of Yak-1 with PF engine is quoted to be 531 km/h at sea level and 592 km/h at 4100 metres, and of Yak-7b it is quoted to be 531 km/h at sea level and 588 km/h at 3860 metres, and of Yak-9 to be 520 km/h at sea level and 599 at 4300 metres.
The P-39D-2, K and L versions are slightly faster at altitudes of and below 3km where it can operate the engine at 51" Hg (which all of the above were cleared to do), and were about as fast as the Yaks on military power at 4km. Overall I would rate it as somewhat superior, especially given it was superior in the dive, which is quite important, and had better visibility to the rear until the advent of bubble type canopies on Yak series.
When we compare the P-39M/N (and also later Q version), there the Yak is inferior. Top speeds on WEP are around 540 km/h at sea level (not much of an improvement there) and around 620-630 km/h at 3km, and on military rating top speed is 610-620 km/h at circa 4700 metres. In addition it will outclimb the Yak at low altitudes (margin depends on respective versions and is not large, but it exists) when they are both climbed with radiator open. By the way, Kwiatek is right regarding the necessity of keeping radiator open in climb.
Yak-3 received a stronger engine and more aerodynamic improvements, while the P-39N saw no further engine improvements, and essentially reached it's peak with the early P-39Q series.
So his comments are entirely understandable.
Edited by Cpt_Branko, 17 February 2017 - 15:34.