Any mod you do to make your car breathe better/easier is a worthwhile one, plus it gives you a deeper tone to boot. The stainless Corsa uses is the best, and will last longer than your car most likely.
If you go with an off road x or h you have the exhaust fumes(no more open windows), have to get it tuned(turn off OS's), buy new AB's (cause it's gonna be REALLY loud), and if you state has emission's your going to have to deal with that too. That's what I could think of off the top of my head :-D
From Corsa web site:
Mustang GT 5.0L 153% Airflow over stock with their AB's, 18 lbs. lighter than stock • +7HP & +4tq
& with xpipe +3 additional HP. 10 HP increase Total.