Well, there is not much of a difference. The stock rubber ones are prone to degrading. When I switched my GT to MM urethane the rubber stockers I removed were shot. It tightens things up a little but not much. On the Cobra I went with the aluminum rack bushings, they suck because there is some noise (steering gear clunks against k-member) especially when backing out of driveways. I have tried urethane, rubber, and aluminum. IMHO the urethane is the best.
Chnaging the bushings is more about preventitive maintenance then performance but you will feel a small difference.
