Comparing Yak-3 to the Yak-9U:
The speed difference depends on the data used. It is true that according to one dataset the difference is only 6mph (351mph vs 357mph). A serial production Yak-3 was taken from the production line for testing and it did 345mph on the deck.
*The power out puts listed are "at altitude", not at sea level.

Both wingloadings and powerloadings are very close to each other. Both use similar props, and identical airfoils (Clark YH 14% at the root and 10% at the tip.) and very similar wing planforms. How it will compare to the rest of the planeset depends largely on whether Pyro updates the Yak flight models in general.