Clark Motorsport have responded in the other thread you posted in btw; http://www.350z-uk.com/topic/78035-new-cats-engine-management-light-on/#entry1242200
Some very knowledgeable garage traders on here, CS, Horsham Developments, Clark Motorsport and Abbey Motorsport just to name a few. Maybe best to try a PM to the garage closest to you for a better response.
High Flow Cats will still bring up the EML light as mine did but you can turn it off by doing an ECU reset (pedal dance) if you like each time; http://www.350z-uk.com/topic/53148-how-to-ecu-accelerator-pedal-throttle-valve-resets/
...or by having the car correctly mapped for them via an UpRev which is the more advisable option as you'll get the best benefits then & it will run better overall. Not sure whether the 02 spacers will stop it but that's a question you should ask one of the garage traders on here. Good luck.