I read in R&T, about the '03-'04, 350Z, this is a direct quote from the magazine; "however, the Nissan spokesman said a change in tire recommendations and alignment specifications does correct the premature tire wear problem."
The later models don't have this issue.
I am assuming this is the same problem with the G35s as well, it is, for me.
Has Nissan resolved this tire wear problem for you?

Premature tire wear or just normal tire wear for soft compound tires.
On average 200 treadwear index tires only last 20,000 miles.

I was of the understanding that the G35 did not suffer from premature tire wear like the Z's since the camber settings were not as aggressive in the G. I have not suffered premature tire wear - my fronts have lasted 16k miles so far and probably have another 2-3k in 'em. I would consider this normal for a summer compound tire.

Back in 1990 the Q45 used ultra soft 160 index Michelins to win braking [129 feet] and handling magazine tests.
US buyers cried and moaned about 16,000 mile tire life.