For a bit more info, the early 986s (1997-1999) did not have secondary cats and were still California emissions compliant. As for mods, there are several aftermarket exhaust manufacturers offering lower cell count, higher flow catalytic converters as part of their exhaust package. For example check out Fabspeed's full exhaust package for a 986 on Pelican Parts.
Also if you're possibly interested in a '99 Boxster, let me know:
'99 Boxster for Sale