99 - 04 MUSTANG V6 PERFORMANCE QUESTIONS

Higher flow fuel injectors are only necessary if you are feeding the engine a lot more air (forced induction).

A pulley can help you pick up some horsepower, but only 10-15. It's a cheap mod that works though, and also helps that annoying power steering pump whine.

A freer-flowing exhaust can also pick up 10-20 horsepower, but it's mostly about sound. You have to be careful you aren't making the system too open if you have stock induction, since the V6 needs some back pressure to work efficiently. Too much exhaust can actually cause you to lose power and gas mileage.

Drilled or slotted rotors are purely cosmetic. You'll never be using the brakes hard enough on the street to notice any improvement in cooling. Stick with the slotted rotors if you want them- the drilled rotors are known for cracking.

A larger throttle body might give you a few extra horsepower, but only if you already have an aftermarket intake.
Your car doesn't have a carburetor.

cross drilled and slotted rotors also eat pads