Slice, I know you wrote this original ques a year-and-a-half ago......but I also have a G37 vert with the staggered set of tires in the (non-sport) 18" size. Let me say that 255 on ALL FOUR corners is odd because that is a good bit too wide for the front rims. You might not notice it, but your steering may be slower lock-to-lock and the handling might suffer slightly.
I MAY be wrong, but I thought the stock sizes for 18" wheels was 245 in back and 225 in front. I believe this because I know I "upsized" both front and rears ONE SIZE: I went from R: 245 up to 255, and F: 225 up to 235. I'm surprised a tire installer would cram a 255 tire onto a stock rim that came with a 225? You might have some tire-tuck at the corners that affects your treadwear as well.
If you read this comment....I would 'run those out' till your next replacement and then return to the stock or near stock widths at both ends....I believe the stock 18" rims are different widths front and rear....so having same width tires doesnt even help the 4-tire rotation since you wouldnt want the NARROWER front rims on the rear of your car anyway!
I had 2 sets of staggered tires in the stock widths (245 and 225) and 2 sets in the next wider widths (255 and 235). I like the looks of the slightly wider tires and my G37 runs and handles as good or better with the latter setup and sticky Continental Sport tires.
Good luck...I hope you are enjoying your car!