About how much would hp would I be gaining, my cat is so bad that it will not rev past 3000rpm, so I imagine especially for me this would be very beneficial. With just a standard muffler would there be enough backpressure to not cause unforeseen issues?

 

If your cat is seriously plugged, or restricted, you will see an improvement in power, if it's not, you won't.

I have run one in the past with stock exhaust and gutted cat with no issues.