Ok, you two situations here....
1. Yes, the suspension is a lot more stiff than the "normal" 370z. It is a very stff ride. If you drive daily on bumpy roads, get ready to hold the twins.
2. There have been some members on these forums that have found out that the dealership has failed to pull out the spring spacers out from the front springs. When the Nismo is shipped, Nissan installed 2-3 spring spaces per side of each of the front springs. They did this so the nose of the Nismo doesn't scrap when its driven off the semi trailers during US deliveries. Bottom line, the spring spacers may have been on the springs when you drove the car which would have caused the extremely stiff ride.
If you go and visit the car again, turn the steering wheel all the way to one side and take a look at the front springs. You might need to rub your hand around the back side of the spring as well. Sometimes the Nissan dealership takes the outside facing spacer off but forget the pull the back facing spacer off.
There is a thread on this and I will try to find it...here are some pics of what you will be looking for...
ATTENTION: ALL Nismo 370Z Owners: Please check your suspension!