thats a lot of money to replace the cat. i bought a no cat exhaust with flowmaster 40s on it second hand on CL and put it on my old GTP. It sounded awesome but ran horribly without the cat. It was misfiring. So took it to the Quest exhaust shop and they quoted me about the same. I was like "800 bucks to cut off the down pipe put a cat back on?" Well, I had the cat already and the labor wound up being only 100 to cut and weld. Point is, you can just get a cat yourself and put it on yourself because they bolt on factory exhaust. Save a lot of money. The cat was off a parts car and the guy only wanted 50 bucks for it. Ran fine after that. Gas mileage went back to normal.
Just have to jack the car up and have some extensions and I think its a 13 mm socket. Its kinda weird laying under the car like that but if you have it up on stands on the frame of the car and the jack its ok. and the wheels chocked.
If that is actually the problem.....