I have just recently cored my exhaust. I have not removed the cat off the lower pipe. The bike runs excellent, have not had any issues. I will eventually check the plugs to see what color they are. My guess. Nothing.
your front cyclinder would run as lean as your rear cyclinder.
You may or may not notice a little better performance.
I went with a full system and the difference was night and day. The bike runs a bit leaner; over 20k miles trouble free.