Just purchased a pre-owned 2011 G37 convertible base model with most upgrades.Question: the car came with Nankang 255/40/R18 tires both front and back and not staggered width as per Infiniti standard spec. Does anyone see any issue with this tire size and setup. Tires are new so would prefer to keep them. They are a bit noisy but the tread recess width is quite wide which may contribute to this?

I do not see a significant issue with this.

They are ever so slightly less tall (2.3% more revolutions per mile) than the stock tires.
This has the effect of a very mild lowering.

.. they are pretty close to stock and yet wider.
Seems like the previous owner knew what he wanted.

If its what you want, is another question..
you should see a bit of extra lateral grip in curves in the dry but also every so slightly less sidewall for possibly reduced ride quality.

As a man with one foot firmly in geezerdom I would not personally not buy this size... but if finding it on a car likely leave it on.... it really depends what you want from the vehicle.
My advice is to run them until they are worn and then replace with stock size.

PS: I looked but only found 2 stock specs for the convertible... Base: 245/45R18 96V all around and Sport: 245/40R19 98W all around