Going back to the original question of if a wider rear track can reduce understeer. Recalling my autocross days, I don't think so. 5mm is also so little, it is really a fine tuning tool. Keep in mind, this is only when approaching very near the tractional limits at moderate to high speeds though. This is not going 10mph in a parking garage.
The same can also be tuned by increasing pressure in the front tires, removing the front anti-sway bar among a host of other things, see below. Cut and pasted from
EXECstudio.com - Tech: How to modify Oversteer and Understeer
Understeer, Oversteer and how to fix it:
Understeer is common on 90% of the road cars because it tends to be a safer setup for daily driving conditions. Oversteer happens when the car gets loose. In this situation the rear tires slide out first
Oversteer can be dangerous especially at high speeds as it requires good driving skills to keep control of the car.
Understeer Corrections
Raise front tire pressure.
Lower rear tire pressure.
Soften front shocks. Stiffen Bump.
Stiffen rear shocks.
Lower front end.
Raise rear end.
Widen front track.
Install shorter front tires.
Install taller rear tires.
Install wider front tires.
Install narrower rear tires.
Soften front sway bar.
Stiffen rear sway bar.
More front toe out.
Reduce rear toe in slightly.
Increase front negative camber.
Increase positive caster.
Soften front springs.
Stiffen rear springs.
May need more front suspension travel.
Install wider front wheels.
Use softer front compound if possible.
Use harder rear compound if possible.
Remove weight from front of vehicle.
Add weight to rear of vehicle.
Note: This is a sample of the methods used to correct various handling problems. Books have been written on this subject. Not every correction will always work as expected. Stiffer front sway bars will in many cases will decrease understeer because of decreased body roll and better camber control. The best rule is to change only one thing at a time and keep notes.Shock adjustments have the greatest effect on corner entry and exit.