Your service engine light will definitely be on. A lot of people have to get regular emission tests every couple of years, my friend has a 93 Mustang and installs his cat replacement straight pipes 95% of the time and only puts the cats back in at test time. :fire:
There are a few members who run the RT y-pipe, with RT cat-pipe, which is a catless y-pipe and single cat pipe versus the 4 factory cats, plus the RT cat-pipe is 2.5" versus the necked down 2.25" of the factory cat-pipe.
As Scott said, buy a pressure tester. Arbitrarly buying cats is expensive guess work.
If you find it is the cats on the y-pipe. I would go with the RT-ypipe. Not sure about your state. Only a few like CA. look for those. My state inspectors only look for the main cats. I love the rt-ypipe, on mine. Deeper sound and better flow. Looses alot of the raspy sound of the V6.