For a street car, probably not. If you are a serious racer than maybe.
I don't have them and don't plan on getting any any time soon. The most I'll do is replace the old worn out bushings with some from Energy suspension and bust out the welder to box my own arms.
From what I undertand, poly bushings aren't the greatest idea either without a panhard bar or something to control that oversteer "snap". The stiffer bushings just move that snap higher in the MPH range (making it even more dangerous).
The way I remember it being explained to me is that our suspension's high roll center and rear roll bind causes that oversteer "snap" that so many of us have come across (once she starts fish tailing, it's damned near unrecoverable). The poly bushings help to reduce this tendancy but actually end up making the problem worse. It will now require more force in that high center bind to make this happen. The side effect is that we've now pushed the problem higher up on our speedometers where the result is even more devastating. Having a sphereical bushings at each end along with a panhard or watts link located the axle so that the suspension will travel fully (up and down). My fairly limited grasp of the whole concept is what has me looking really hard at either the MM 3 Link or Steeda 5 Link setups. Both are desigend to eliminate that binding we have with our 4 link systems. In a nut shell... if you replace your bushings and nothing else... It may be in your best interest to stick with factory type rubber bushings until you've found a way to positively laterally locate the axle.
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.
You can easily make you own subframes. measure the length. go to a metal shop and get someone to cut you the correct length rectangle tubing. 2" x 1" Take it to a muffler shop and have it welded on. Cheap SFCs. Save yourself 100 bucks after the welding is done.
As far as upper and lower control arms. They are awesome and u will not believe the difference. I got a set of the UPR upper and lowers...they made a HUGE difference.