There is a whole binch of things that can induce and/or reduce understeer.
Try inflating the front tyres about 2-4psi above the rears.
I have a GTS4, and found that a heavy rear bar, combined with the standard front bar, was good for a neutral to slightly oversteering car.
Do you have any -ve camber in the front? Try taking some -ve camber out of the rear.
What spring rates are you running?
Also, try adjusting driving style. If you are braking as you turn in, then the car is more likely to understeer as weight transfers to the front outside tyre.