You know, a factor that no one has brought up is the tires. I'm sure that part of the reason that there's so much pebble noise is because the Z has fairly sticky stock tires - it's how they got the G numbers so high for such a relatively heavy car.
Putting sticky tires on any car will multiply the pebble and gravel sounds tremendously. I'm not saying it's the only factor - Nissan's goal of lighter weight for this car also lead to the drop in sound insulation - but it is a major one. I'll bet cheapo highway tires would cut the racket by 80% (too bad they're only good for burnouts
).