Don't know about noise levels but there wouldn't be any performance gain as the ECU learns any mods and unfortunately counter reacts them. The way around this is a piggy back ECU or something like UTEC.
Noise is pretty important though
Sorry mate, thought you were no longer interested after your heads up earlier to Jim.
one here on ebay
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... PcY_BID_IT
Guy on ebay does them for a fiver all in and straight swap. Couldn't be easier. Just look under 350z on ebay and you'll see one on the side of a yellow gt4
I always wanted a GT but many would say it's over priced for what you get. Also depends on whether you would want to mod the stereo and the seats. If you are going to them don't bother with the GT.
it is possible to do it with just lifting the bonnet. It can be fiddly but it is doable. Drivers side took 20 mins and passenger side took 10 mins to do.