At 17K the Yak-9U is capable of over 400 mph tgs. It will run with the fastest planes in the arena and has a good climb rate. It's got a decent turn rate (better than most fast planes) but not great. It's a deceptively small target, difficult for an enemy to hit and takes damage extremely well (I've continued to stay and fight in it missing an elevator, an aileron, a gun and a hit to my oil and won). It shares the poor ballistics of most Russian/Japanese fighters but if you hit the 2x20's are deadly (keep convergence inside 400 max).
About the only thing to dislike about the 9U is the lack of WEP (try to hold E to compensate) and control stiffness at speed (use throttle to control speed and keep those trim controls at hand).
The 9T is considerably slower and climbs worse but the 37mm's really pack a punch if you hit with them.