Yes it will be louder and yes you will get a code. you should be able to find some
"mil" Elminators in any Jegs or Summit mag. these will keep you from getting any codes
my car has no cats check out the video to hear a sound clip of it.
If you have an XCal II with a canned tune, you should be able to turn off the rear O2 sensors thus eliminating the need for MIL Eliminators. If you have a "custom internet" tune from the likes of BamaChips, Evolution, Brenspeed, etc., they turn the rear O2's off on all their tunes so you won't get a code with their tune either.