Mach-fly, the less hp is saying basically the same thing as less speed.
Main differences:
Reduced top speed
increased climb (and possibly acceleration)
Reduced range (not usually an issue)
Like I said, you trade performance in one area for performance in another, but neither of the changes will really give it a big advantage or handicap in a fight, much like the Yak-9.
Personally, I think that the biggest difference might come in the form of handling characteristics. Might be a bit harsher than the Yak-9, might be more lenient, might be almost the same, I don't know.
But the fact remains that the yak-3 is in no way 'needed', or even very high on the priority list (based on potential uses) for MAs OR special events. However, or EW and MW GV setup is pathetic and lacking, respectivly.