Re: cat converter
Physically removing it is quite easy...dealing with the consequences, not so much.
I believe you would throw a CEL unless you can turn off some of the system monitors. I am not sure if this would force some of the control systems to go into an open-loop mode or not, causing detriments in performance and fuel economy (some CELs cause this, others do not). I also believe you are more likely to burn through any downstream O2 sensors.
If you have any emissions related testing in your location, you will fail.
This is of course in addition to the needless harm to the environment this would cause, with very little gains (or perhaps losses) to performance.