No experience on a Z, but I had a Designtek exhaust on my Boxster, which uses a Janspeed outer and reworked internals. Quality was nice enough.
The two I have on my shortlist for the 370 are either a Berk CBE or the Borla - both look fairly OEM too.
I know Chris has the Stillen, which sounds lovely but personally not a fan of the sticky outty tailpipes.