I had some trouble with the ignition on my EL XR6 5 speed, which has a cat back 2.5" system with intermediate and rear mufflers (no front resonator), and a lovely iron hand in velvet glove subdued note. Until the drama it had never been near a dyno.
To confirm all was OK I put it on a friend's dyno. The operator ran up the car, studied the screen, scratched his chin, checked a few parameters and quizically asked what I'd "done" to it. I said nothing, apart from the ex system. Very surprised, he informed me that I was getting 132 kW at 150kM/hr in 4th gear on the rollers. He recorded the figure as the best he'd seen on a car of my type.
I have no idea what I was getting before, but I'm not unhappy at all with the output. I do think the exhaust helps, but I have no way of verifying to what degree.