for some reason I thought you had a 4cylinder 320, but re-reading your posts seeing as you have an E91 325i with N52, yes the cats are built into the exhaust manifold - could of told you before if I realised lol.
If you want OEM for piece of mind that's cool, but I've already voiced my thoughts in that an exhaust shop can do you a quality piece of stainless steel without the BMW logo for a decent price to replace your centre resonator.
I'm not too familiar with the difference between US and Euro spec performance exhausts though.
Does the US version include the midpipes? Does it fit the E91? if so, then you are good to go.