Do these really add HP? I have a lightly modified GT Auto (CAI/UDP/4.10's) and was thinking about these. Anyone ever dyno before and after install in a naturally aspirated environment?

 

Don't bother with them until you have intake/headers/full exhaust, and have done all the other beneficial bolt-ons...and even then, don't bother with them.

They do show some gains if you do a custom dyno tune after installing them. However, they are really only a hinderance after boosting your car.

 

I thought exhaust would be minimal gains 4-5HP (and I like the "quiet" stock sound). Although the Borla sound is intriquing.

 

Changing only the mufflers is what many people consider "exhaust". This changes little other than the sound on the S197 Mustang.

The real improvements in airflow are acquired by proper-designed aftermarket headers and mid-pipes. Tuning for these results in even greater gains.

Think of it this way:
You have a hose with a nozzle at the end. Changing the nozzle will change the way the water sprays out, but changing the hose for a bigger/better hose is the only thing that will increase the amount of water flowing out of the nozzle.