Would swapping the 3.3 with a 3.5 from a 350z be a good middle ground compared to the 3.7? Sorry still trying to understand these things. If it decreases the life of my G substantially I'd rather not

If your car has the sports package (aluminum pedals, sports bumper and side skirts, orange stitching on the steering wheel) then you already have a 3.5 diff. I would look for a 370Z/G37 diff since they are 3.7 and would probably have lower mileage on them. As long as it is from a manual which have a 4 bolt flange, it will bolt directly onto your car. Going from a 3.5 to a 3.7 is a small change but still noticeable. I would not recommend a 3.9 for a DD since the 5AT is already missing a gear. Not sure if the pumpkins from the newer Q series are similar to ours but that is something I would also look into. Your engine will be fine with just a simple final drive change. The Transgo VB is a great mod for the autos. As already mentioned, a tune won't be needed for a few bolt-ons but it will help. If the suspension is stock then sway bars will make a noticeable improvement.

Well I'll be. In all my reading here, this is the first I've seen this info. But it sure does make sense. That explains a lot about why my MPGs don't ever quite measure up to what others are reporting! Thanks for the nugget....

I think all 2006-2007 auto coupes got the 3.5 diff whereas all 2003-2005 auto coupes got the 3.3 diff, regardless of optional equipment. Easy enough to throw a match mark on the tire and driveshaft and count revolutions. I thought the orange stitching denoted the sport package which meant VLSD.

EDIT: I know Z1 Motorsports has a link on their gears page to a chart that lists all possible combinations of chassis/transmission/gear ratio.

I discovered that years ago when I was swapping pumpkins. It also mentions it in the FSM.