I can't speak for the Mac's, but there are are exhaust systems (Long tubes, high-flow cats, and better flowing mufflers) that have been shown to produce gains of over 23HP and 28LbFt of torque on what should be (no other mods listed) an otherwise stock 05 Mustang GT.

 

The full Mac setup sounds soooooooooooooo Damn goodI am very envious to say the least[&o]

 

How is the fitment of them. Does the o/r prochamber give off a really gassy smell?

 

Any o/r pipe will smell like gas.. To the OP....your car will run better. Hp #'s won't give you the smile these babies will put on your face. Don't break that piggy bank because you will want a blower.

 

Should sound sweet! Please post some sound clips when you get that mac setup on...can never hear enough of those things

 

Should be 15 HP at the wheels. Assuming long tube headers and 3" pipes.

 

15-20RWHP

and yes you should be over 300RWHP with the mods