I have actually went from super duper cat to a Decat. Yes you only gain 5kw in the top end. With my car I gained 30rwkw in the midrange. That's a huge difference. Boost comes in quicker and the car feels much better
As for the 10,000 fine. You would gave to be the biggest drop kick in the world to cop that fine. Only the EPA can fine you with that. And if you rock up to a EPA inspection with no car you deserve it. I have a 3 inch pipe going through a cat shell. Still looks like I got a cat there