Just some thoughts C-Hing.
Did you check the left and right weight balance? Turn in differences are caused by different reasons for low speed and high speed, and driving style. In general, low speed difference is more affected by structural geometry (steering angle, toe in out etc.); whereas high speed difference is more affected by the whole car (including weight distribution).
There can be expensive and cheap way. There are tools to help check overall weight balance of a car wheel by wheel but costly. Some C-Hing use 3 block of books plus one electronic scale on each corner of the car, all set to be about the same level (which we can use a decorator spirit level to check), and then measure each wheel in turn by swapping the 3 books and scale one wheel at a time. And then you can record the wheel loading of the 4 wheels and see the difference.
Typically, you can then add weigh or shift things about to level load the wheels, or some C-Hing would adjust the spring tension on the shock absorber to increase or decrease load of the wheel that needs to be adjusted.