I have a 04 mustang GT is it possible to cut the cats off without it messing anything up performance wise

 

You can remove the cats and it won't hurt performance, it will actually increase performance by allowing the exhaust to flow more freely. However, doing this will cause the O2 sensors to throw a code. It will also cause you to fail emissions test if your state requires it. You can stop the code with a tune, or mil eliminators.
Another option is to get a good X/H pipe with free flowing cats, or even an off road mid pipe.

 

I already have a H pipe in the car from the factory but if I just replace the O2 sensors with new ones or dummie O2s would that work for the code issue

 

MIL eliminators are O2 sensors that trick the computer(Won't work after 2004 MY). I think you can just use spacers on the ones you got to back them out of direct contact with the exhaust(Won't work after 2004 MY). Your absolute best way is to just get an SCT tuner from somewhere like AM, the tune can give you up to around 20 extra HP and you can tell them to turn off your rear O2 sensors.

 

Ok yea that sounds like the better option how much do those tuners coast?

 

http://www.americanmuscle.com/x3-cus...er-9904gt.html

$380, free shipping, lifetime tune upgrades if you ever get new performance parts, and comes with 3 tunes loaded for your car and parts.
Best upgrade you can buy lol.

 

That is a pretty good upgrade ha ha little expensive but lifetime tune upgrade makes it worth it