I recently came across some really good quality High flow cats made be Berk Technology. They are "Brand New" (still in box not open yet) for the '07 and up Nissan 350z/370z/G35. Asking price is $500
Here are some details form the website about them. If you want pics email me at email@example.com
"Metallic substrate spun catalytic converters, OBDII & 49 state compliant. Full stainless steel construction. Metallic substrate offers higher flow and lighter weight. Uses advanced spiral wound ultra thin foil. Includes our custom made soft crush graphite collector gaskets for a leak free seal"