you can keep a max turning speed if required, but there's no reason to force accel as you approach that max speed. and there's definitely no reason to have negative accel once you hit that max speed, it should just stay flat/capped.
Ideally a graph of in game turning speed (y) vs mouse speed (x) would have a straight y = x line till it reaches the maximum turning speed determined by your currently equipped items and then it should stay a flat line out past x > max turning speed.
currently it's some weird bell curve.