You'll probably struggle find someone to gut a cat for you, because it is illegal to do so.
Most people do it themselves. A guy I know told me that it is very easy to do after the cat has been removed.
I have of course never done this myself.
I have heard that gutting the cat will cause the engine to make more power. It should be noticeable.
I'm not sure about the CEL, as I don't know much (anything) about the Spyder.