My car was meant to have a full 3" system, on closer inspection it had the standard front dump, 3" dump, standard cat and 3" cat back, no idea why ppl would do it, its just a waste of money!
dont gut your cat you made need it in the future, just put it aside for when you may need it.
The cat you get depends on how much power you want, i have a jjr xforce, was fine for 200rwkw but now im looking at pushing it to 300rwkw its time for a new one.