Has the car always had an understeer problem (like even with the previous set of tires)? You might also want to try changing your swaybar setup. A bigger one in the rear, or a smaller one in front will give the car more oversteer and even it out. When I first started drifting my Camaro I always had a problem with too much understeer. Most cars come from the factory with the tendancy to understeer. So I slapped on a bigger rear OEM swaybar I got at the local yard for $30 and it fixed my problems.