The underbody aero kit (as far as I know) does not change the ground clearance distance. It does make the front spoiler longer by a couple inches.

If you get the splash guards, some people have reported scrapes, but they can be easily replaced and are inexpensive.

Unlike some other sports cars such as the Camaros (and similar body style cars), the G35/350Z cars front wheels are closer to the front bumper. Getting scrapes from driving up a steep driveway or going over a speed hump is usually not a problem with the G35. I do know the bumpers are low and will hit many curbs and parking blocks.

The pics shows the distance from the wheels to the bumper. You can see the Camaro Z28 (sorta of a bad angle) has a longer front end than the G35/350Z below. Both of the G35/350Z cars in this particular picture cannot make it over the sidewalk in front of them without destroying the front end. Make sure if you let anyone borrow your car that they know not to use the curbs/sidewalks as a guide to pulling the car into a parking spot.

322133-98_camaro_z28_350z_G35.jpg