My experience with 280Z's (not the ZX's) is that the 2.25" exhaust produces less resonance at the lower rpm's in the passenger compartment than a 2.5" setup. I don't have any experience with how the CAT affects this resonance, and it may have the same effect as a pre-muffler or glass-pack. Also, I would think that unless you are going to perform major mods to your engine a 2.25 mandrel bent exhaust should have plenty of flow.
I just got the Monza exhaust from NOPI. Yu can see a pic of it in the Vic Brit catalog but it is cheaper from NOPI. I got the headers too. It will be installed in about two weeks and I will post as to how they fit and sound.
On my 83 ZX (na) I changed the cat to a high flow unit then used 2 1/4 pipe back to a Flowmaster Delta Flow two chamber muffler. The sound is not outragious, but I did install Dynamat around the back wheel wells and the hatch area. The muffler fits and is out of sight.