RCZ's 370Z Journal.

RCZ

Rears weigh 21.3lbs which is around 2.5lbs lighter than stock rear. Fronts weigh 20.6 which is 2.6lbs lighter than the stock. Not bad for being a half inch wider. New lugnuts weigh a bit less than 1/4lb per corner vs 3/4lb per corner of the stock lugnuts.

That adds up to 10.2lbs for wheels and 2lbs for lugs for a total of 12.2lbs. Now I've heard estimates as to what that translates to in sprung weight. Usually you multiply by 4X. So thats ~50lbs of weight from the stock setup for half inch wider all around

EDIT: ALSO

Stock tires weigh:
Front Stock Bridgestone RE050A (245/40/19): 29lbs (13.15kg)
Rear Stock Bridgestone RE050A (275/35/19): 33lbs (14.97kg)

The setup I want to run:

RE-11 265/35/19 - 29lbs
RE-11 305/30/19 - 32lbs

That is an additional weight saving of 2lbs even though the tires are 20mm and 30mm wider.