Not sure where you heard that the Track model would be lighter, but you can find the specs here;
http://www.ragusa.cc/public/
Same info I got in the mail from Nissan. According to the brochure here are the weights of each.
Model MT/AT
350Z 3188
Enth 3197/3210
Perf 3217
Track 3225
Tour 5AT 3239
Tour 6MT 3247