the rear roll rate will have changed dramatically with the different spring and damper rates. as the front is (or should be!) connected to the rear via the unitary body construction, this can have a major affect on the front suspension genometry during cornering, which could be why understeer appears to have reduced.

the different angles of the trailing arms can also have contributed to the degree of camber, toe and castor of the rear wheels when cornering, maybe giving a more positive 'rear steer' effect that helps the car turn in.