There were 2 versions of the Yak9U:
1) The first 9Us were powered by the VK105PF-3, very weak engine carried over from the -9D.
2) From August 1944 onward they were powered by the more potent VK 107A, which gave it a max speed of 434 mph at 16k and 379 mph at SL... after flying for a bit at 16k, I could never reach anything close to 350 mph, max I was getting was about 300 with 75% fuel.
So it seems that we have the VK105 version, which doesn't make much sense since more were produced with the Vk107, still, however, even with the 107 the 51 should still be faster than the Yak at all alt except for maybe 10-20k.