Indeed, the slats on 109 wings were able to make the aircraft turn better according to the Germans, and planes like the La-5 and La-7 have them, but I've never noticed them in-game. The cause for the better turning ability is that the slats (according to the diagram below) seem to cause a Bernoulli effect, pressurizing and speeding up the air over the wing. This delays separation of airflow and maintains lift at higher AoA.
I don't know why others haven't noticed before, but I've mentioned it a couple of times as well, kozzm0.

There's a lot of error in speed and acceleration; a major problem in an energy fight. Like you said the Yakovlev should pass 700 km/h in level flight, and my Spitfire IX should easily pass 320 mph @ SL. I can't even pass 305 mph. Our inability to accelerate from slower, lower-powered foes makes it nigh impossible to truly energy fight since energy fighters generally have the ability to gain energy faster than their opponents. Since there is no such advantage in BoP, there is nearly no way to really energy fight. Simply we can only extend, climb and BnZ slower aircraft rather than gaining angles on them using superior speed gained from faster acceleration.