Originally Posted by Abdiel

Almost all of the aftermarket Y pipes including MD XYZ are 60mm or 2.5in at cat side and 70mm or 3in at mid pipe side. The only one that's not is the ED Y pipe which is dual 2.5in to single 2.5in.

OEM Y - 1.75in + 1.75in -> 2.5in
MD XYZ - 2.5in + 2.5in -> 3in
Tanabe Y - 60mm + 60mm -> 70mm
ED Y - 2.5in + 2.5in -> 2.5in

This thread has some discussion that may answer some of your questions:

Motordyne XYZ-Pipe (Pilot Production) - MY350Z.COM Forums