only sound difference you'll hear is a slight spool of the turbo in your car. You can hear it slightly from the outside but then again I have a straight pipe second cat so mine sounds a little clearer.
your car is gonna throw a P0420 Catalyst System Efficiency Below Threshold (Bank 1) and some other code.
despite having a tune, you will have this code pop up but if you're like a few people and myself that don't really care, the software allows you to simply turn it off. What you do is simply go into the software while your car is connected like if you're about to tune it, then go down the list to find P0420 and uncheck it. Once unchecked, reflash your tune and boom, goodbye CEL for the downpipe