The rear end on our cars is so loose, even in moderate loads the rear axel tends to move around and this causes a rear steer condition, which is why the vehicle feels so unstable. You can actually move the rear end back and forth by hand with the car on a lift.
Poly bushings might help some, but in a stock controll arm I say it's a waste of time, money, and effort when aftermarket units are so much stronger than the stamped steel units from the factory.
The 4 link suspension is great for drag racing, but not for cornering. A panhard bar setup is much better. If I were to make a Mustang handle with a solid rear axel, there is no doubt in my mind I would go for the MM Torque arm system. I called MM once when I wanted to make my car handle better, they said that the Panhard bar system would make a huge difference.
I have HPM Megabite LCA and UCA's. These are drag units but they made the rear so much more stable I couldn't belive it. With a RWD vehicle, you use the front wheels to point where you want it to go, and hit the gas and the car should head that way. With a good setup, and knowing how to use it, you can really corner well.