Out of the factory these cars all tended to push through turns with the factory parts installed. This is why adding a bolt on rear sway bar tended to help neutralize them and significantly improve their feel.
Replacing the link bushing with poly shouldn't necessarily introduce an understeer condition, but if it didn't exist in the same degree prior, than it obviously impacted something. All I could think of is you have improved the reaction and application of force in the rear of the car and it is overpowering the front shocks ability to control the force. If you have wider rear tires than front, the more direct translation of force could be creating more grip at the rear now, which will also overpower the front tires.
To offset the push, you could add a stronger front shock, larger sway bar, or thicker T-bars.