If the CAT looks like this then it would free up exhaust flow but how much power it would gain I am not sure.

This old style of CAT had ceramic balls in it coated with platinum/iridium. They choked the exhaust pretty bad.

Later styles up to today have a honeycomb matrix and flows much better and restrict flow very little, assuming it is not clogged or clogging.

As for the O2 Sensor and EFI. EFI systems require at least one O2 sensor before the cat. The cat cleans up the exhaust and the O2 sensor for the EFI needs to see how dirty or rich the exhaust is to adjust the air/fuel.

The O2 sensors after the cat (95 and newer) have nothing to do with the engine running. They are there to sniff the exhaust and see if the cat is working. If the reading before the cat and after the cat are the same the cat has failed.

If you want to delete the CAT and stay stealthy cut it open and weld a tube through the middle. I do it from the top and weld the top back on. If you don't put a tube inside the cat housing it will have a definite ping sound as the exhaust echos in the hollow cat.

If you remove or gut a cat on a 95 and later system with 2 or more O2 sensors put a spark plug non fouler adapter or O2 Spacer between the REAR O2 sensor(s) and the exhaust. It pulls the rear O2 sensors out of the exhaust flow and tricks the computer into thinking the cat is still there and ok.

Hope it helps.