The point is, you shouldn't worry too much about teacher performance.
If it's only loosely correlated, measuring value-added then punishing / rewarding teachers based it won't be very accurate. It's like lines of code - programmer talent is probably weakly correlated with lots of lines of code, but you don't want to reward or punish them based on LoC because it will lead to pathological behavior.
You should look at other things to alter (which are less easy to game once you start trying to control them), like class size, course materials, assessment style, how the teacher actually teaches, etc.
If it's only loosely correlated, measuring value-added then punishing / rewarding teachers based it won't be very accurate. It's like lines of code - programmer talent is probably weakly correlated with lots of lines of code, but you don't want to reward or punish them based on LoC because it will lead to pathological behavior.
You should look at other things to alter (which are less easy to game once you start trying to control them), like class size, course materials, assessment style, how the teacher actually teaches, etc.