Decat just drops the Cat itself, a "straight through" drops the mid silencer.
Doing that can help flow at the top end (for tracking and racing) but can also seriously start to affect back pressure and therefore low end performance, and ultimately make daily driving a bit of a chore.
It will also be conciderably louder, and probably get you some attention from the boys in blue.
Ive ran N/A cars before in such a way, and my personal preference is to decat (or sport cat at the least), but keep a mid box, even a small one is better than non at all.