It's OK to derive CLmax from the clean stall speed. Then for any given speed you can work out how many g you can pull. You can use the cruise CL to derive the induced drag, taking reasonable approximations for the efficiency factor, say 0.9 for a typical WW2 fighter monoplane. You get the total drag from the power, prop thrust and weight and the induced drag from the appropriate formula, giving the form drag (it has various names but its the part of the drag which varies with rho V^2) by subtraction. From these numbers you can approximate the total drag and therefore top speed, climb, all sorts within limits of the approximation. Yes, even ideal turn performance. But not in real combat, where other factors come in.
One result of all this is going to be that if you have need a high CL, you will tend to have a large CDi, and if you are using that in a turn it means a lot of drag, hence deceleration and/or loss of height. With the 109's slots it means good instantaneous turn but not too good sustained, the slats are like parachutes. But the higher AoA means you can maybe get a firing solution.
(For rule of thumb comparison, use span loading for a pointer to how hard the wing has to work to deliver lift.)