Welcome aboard Mat.
Really the only way you're going to be certain what power the cars putting out is by getting it on a rolling road.
First things first though imo ~ check the service schedule here: http://www.350z-uk.com/topic/31110-service-schedule-for-a-350z/ ) ..to ascertain whether everything's been kept up to date on the servicing side of things. It could be that your Zeds in need of some new fluids, spark plugs etc.
All of which may help improve the car if only slightly making it run better.