I think the OP is referring to steering rate. It is not linear as the amount of lock changes with the speed of the car and it affects the DS3 as well as the wheel I use, the DFP.
For the DS3, it can be improved by changing the controller sensitivity to 7, but it the amount of lock available is still an exponential curve based on the speed the car is travelling at.
There really needs to be an option to turn off this aid for those that want to, so a 20 deg movement in the controller results in a (for example) 20 deg movement in the wheels, regardless of the speed you are travelling at.