I have a 1993 WRX with a chip that runs 16.5 psi boost on 97 octane fuel, the chip was set-up to take advantage of a full-decat but i only have a CAT back system.

first of all will i be doing any harm running a full de-cat chip with a cat back system?

secondly if i did get a de-cat down pipe will this destroy my fuel economy?
before the chip i was getting about 23-25MPG now im getting about 25-28mpg which is very good so i dont want to lose that if i dont have too?

any advice or experiances would be apreciated, cheers.
GAZ