What do you mean by a little off? If you are talking about circumference, my Z with the set up I described reads about one mph off (low) on the speedo when comparing it to my GPS, which I am very confident is almost right on the money. I personally think that a 315 would be too wide for the stock 10" wheel. I think at the 305 looks good, but I always thought that the 275 on the 10" looked too small, meaning that the sidwalls angled in too much. The 305 is just about perfect.
Here's a picture of the rear stock wheel with a 305/30 Hankook tire.
http://www.the370z.com/members/azroa...rear-wheel.jpg