No emissions testing means you don't have to replace your cat when it fails. If you intentionally mess with emissions equipment (especially CCs), you can be fined huge amounts (as stated earlier).
Catalytic converters wont affect your fuel economy, they burn hydrocarbons (fuel) downstream of the primary 02 sensor, which your vehicle uses to tell how much unused air is in the exhaust system.
Cats don't have significant performance reducing powers. They resist flow some, and probably limit you by 1-3 HP between 5000 and 6500 RPM. How often will you see that? I did put a 2.5" hi-flow cat on my turbo 6 powered offroad rig, but it revs to 7K. And I would never cut it out. Too easy to see underneath it!