What has been said is correct, I'll add this to why though . . . . the stock control arms are stamped from thin sheet metal and they will flex and twist very easy. The bushings used in the stock control arms are made from rubber which will allow movement fore/aft, side to side and will also twist.
The problem with all this flexing and twisting is that when any force, accelerating, braking or cornering are applied to them they flex/twist so bad as to not allow constant or consistant tire contact with the pavement, thus reducing traction.
All of the aftermarket contol arms are made from strong tubing or solid billet stock, either in aluminum or steel and have hard or even solid bushings which will not allow the flexing and twisting. The rewards are of course your tires staying planted to the pavement giving your Mustang more of the much needed traction for quicker take offs, better stopping and faster cornering.