never heard anyone with just cats and oem zorst mate, so can't help mate
But i'd say you should be still within the limits, maybe?? as the measurement is @ 4000rpms isn't it?
it all depends on what you do with the Z really.
Pads: prject MU are really good too, if you listen to my italian friends over at their Z forum, quite a few track nuts over there
no probs mate, a guy round the corner from me has an 06 too, he must be working for a dealer as he's got diferent nissans all the time, but he now seems to prefer the new silver 06 Z will have to go around and introduce myself