Manufacturers advertise that their cat-back exhaust systems can gain 15-20 rwhp over stock. I don't understand how this is possible if the cutout in my factory SS system only netted me 8 rwhp and 15 rwhp on my 94. How is it possible for an aftermarket system to gain more than what a cutout provides.

Borla and the loudmouth are the only systems I can see backing up these claims because, in reality, they have their own versions of cutouts.

 

They can't. It's advertising, so you can't believe anything you read!
Actually most manufacture have documented these gains. But it's been under very favorable conditions. The average guy on the street seldom see's the advertised gains.

 

That's the difference between RWHP (rear wheel horsepower) and BHP ("brochure" horsepower).

 

LOL, usually a factor of x2 or x3

(A degree in Calculus is required to compute the factor for spiral airfoils and electric turbochargers)