Full exhaust involved changing everything to do with the exhaust.
I.e
Headers -> Cat -> Piping -> Muffler

Cat back is just everything after (and often including) the cat.
I.e.
(Cat) --> Piping --> Muffler.


If you just change the cat and the piping + muffler you might get some gains. But they won't be as much as changing to a FULL exhaust.