damage to your ears over time from the louder sound (and the louder sound system setting)
and
damage to the atmosphere as you add more pollution to it.
Plus I know someone with a dyno who went through dozens of combinations of exhaust component setups before they found a combination that actually gave gains over the entire useful RPM range. It isn't as easy as just taking one from column a and one from column b because their characteristics may cancel each other out.
If I were to want more sound, I'd use Pedro's mod as it is cheap and reversible in a minute by inserting a plug. So if it turns out you don't like it, you can retreat. Or, if you are in an area where such sound is inappropriate (a funeral procession or ...) you can quiet the car to stock.