I can't say anything about the domestic cats. I have a Nissan 370Z that installing a high flow cat or ,like I did, test pipes nets you an extra 10-15hp to the wheels. It is about the same gains as installing a new cat back exhaust. Maybe Nissan just uses horrible flowing cars. My friend has knocked the cats out of 2 of his Titans and they both had noticable gains. I can't say what the actual figures were but we aren't emission tested in OK.
There are lots of diesels that run around here and lots have no cats. I know that the newer ones have problems if run with out cats but I don't know years. If it were mine I would find out when it got to be a nessesity. If yours is safe (I think it is) get some test pipes to replace your cats it you can. That way if you have a problem you can go back to them. My test pipes were $100 as aposed to 300+ for HFC's.