Check out my recording of XYZ Pipe, Japspeed De-cat - standard back box.
http://www.350z-uk.com/forum/viewtopic.php?f=31&t=38181&start=15
Okay the wind noise doesn't help, but rest assured, the sound is fantastic. Superb between 1-3k revs, hit 3.5k and it starts to pick up, hit 5k onwards and it sounds perfect. The next best thing is the motorway. Cruise anything up to a ton and there is no change, hit anything higher and sound increases, although it's nothing more than the noise made by the tyres which is more annoying.
I'd recommend the Plenum, XYZ and De-cats. Mark recently installed these mods on my car with dyno charts on another thread. The other added bonus is you don't have to fork out for an expensive back box. The perks of performance and sound are just perfect.
The de-cats are just about settling down now that I've covered a good a thousand miles, I can't complain in the slightest.