Take the tune out and see if you have the grunt back. From reading the posts on here it seems that sometimes the predator tune actually decreases power on a even slightly modified goat. You might have to get a custom tune done.
The Magnaflow system is designed around working WITH the OEM calibrations. A severe deviation from these calibrations may lead to undesirable results. The Preadator 93 octane tune may actually be introducing knock at low RPM.
Time to do some logging and adjust the fuel curve is the "custom" section of the predator. I have installed several of the exhaust systems and in no one case was their a feeling of lost performance.