Doing the setup for a car is a whole combination of all the parameters.
Let's say parallel steer will help you to have the car behaving as you desire. Having the car behaving as you want can make it feels more accurate ... driving can be very subjective as everyone is not necessarily willing to have the same behaviour for the car in all the possible situations. For sure the general setup inputs (ARB, camber, tire pressure, springs, dampers) will play a higher role in the capability of your setup to be quick around a track.
Being quick in LFS is a big matter of practice combined with understanding what you are doing ... usually after some times building setup in "blind" mode, it is a good idea to take the setup of a fast guy, lapping a lot with it, and then to begin to modify the "fast" setup slightly in aim to see the differences brought by those changes ... after a nice amount of hours you begin to understand what makes you fast and what is not ...
Take note too that understeer and oversteer can be a nightmare as it is the case in RL : a slight understeer you may not notice may lead the car to a crazy oversteering behaviour when back on the throttle ... correcting the oversteering behaviour will then make the thing even worse => in the end you never correct the issue and you loose your trust in the car/yourself ...
As you may have already heard : practice makes better. I would add, without understanding (reading LFS manual is a good start), practice can be very long before making you better