The guys at Pro-Dyno in Hope Mills, NC recently installed and tuned a full Stainless Works Exhaust & JLT CAI on a 2015 Mustang GT. Editor Turner has the full article with a dyno/fly-by video on the Front Page. Click This Link To View It.
Not necessarily. SW flows better than the stock system so you will pick up a very slight gain (nothing noticeable) over the factory catback. The car in the vid picked up nice gains from the intake, headers, and tune. If you run longtube headers though the SW catback will hold back some potential gains. If you plan to run longtubes you may want to choose a different aftermarket catback but if you plan to stick with factory headers and cats, the SW catback system will be a good choice for sound.
The SW Catback is cutting power period. The full Kooks and American Racing setup are in the 440 to 460 range on a Dynojet. Take the catback off and just run the SW headers with cats and you're in the 440+ range as well. It's been tested by owners on other forms.
According to the threads you read on mustang6g...
It's pretty rediculous with these companies charging over 1000 for cat backs because of the so called RnD. It is exhaust tubing and not that complicated.