As I said the rear
control arms connect the rear axle housing to the chassis. Ford, like all manufactures today are concerned about reducing NVH (noise vibration and harshness) as a result the rear
control arms are flimsy and use soft, compliant rubber bushings. Under hard acceleration they flex and do a terrible job of planting the rear which causes poor traction and axle hop. Aftermarket arms are stronger and use much stiffer bushings which improves traction and eliminates axle hop. You can take it a step further and also buy
LCA (
lower control arms) relocation brackets which changes the angle of the arms and further improves traction. I installed these on my '12 Boss:
http://www.americanmuscle.com/bmr-lca-poly-0512.html
However just about any brand of aftermarket arms will be big improvement over the stock arms. I usually buy from AmericanMuscle.com but you can buy them at any of the mustang suppliers such as Late Model Restoration, CJ' Pony Parts, there are others.They are all reputable and the prices are usually pretty close. I installed them myself on my back with the car up on jack stands. Just go on Youtube and watch a video of the installation and decide if you can handle it yourself .I know you are thinking that theses will not increase your horsepower and and your right, but you can spend an "assload" of money and build an 800hp engine but if you can't get that power to the pavement you will be wasting your money and your rear tires
Nitrous probably gives you the most H.P. per dollar, if you go with a high horsepower system, but for a high quality kit with everything you need to install it and make it as safe as possible you're still spending $700-800 or more plus installation plus $50-$60 bucks to refill the bottle. Even though today's systems are much safer for the engines then they were in the past, there could still be engine durability issues especially if you use it constantly, which for some reason I feel like you will. lol . You also need to make sure there is some place near you that can refill the tank. Then there is your safety and the safety of others to consider. When your instantly adding 100+ HP at the push of a button, it very easy to lose control of the car.
Two more things, since you bought the car used there is a possibility that some mods have already been done, so try to figure this out before you start buying parts. Second, is to figure out how the car was equipped from the factory. For example if the car was ordered with the optional 3.73 rear axle ratio I wouldn't touch the gears. However if it has the standard 3.31 rear you might think about an upgrade. You can enter yout vin # here:
http://researchmaniacs.com/VIN-Number-Lookup/WindowSticker/Ford.html to see.
Good luck