As you already have the Cobra Y the Cobra system will work in tune, resonated is a great upgrade for sound, and if you like loud, then the non resonated is the one to go for, but as a rule, resonated is the more popular choice...
For quality and sound I'd say the Scorpion or Cobra resonated system, we do both here - http://cougarstore.co.uk/index.php?main_page=index&cPath=1_10
Drop us a call or PM if we can help...
Have a look at the Rota GTR wheels that we offer, similar in design, photos can be seen here - http://www.350z-uk.com/topic/32711-rota-gtr-wheels-–-350z-fitment-from-cougar-store/
We had an RS4 in the garage today, now that was a sound...
Don't rule out the Cobra Resonated, lots of roadster owners have them fitted too with no issues of them being intrusive...