Looking for new tires for my wifes G37s sedan. Keeping the factory 18" sport wheels but wondering if anyone is going to a larger/wider size front/rear other than the stock 2255018 and 2454518.

 

The problem is the 7.5" wide front rims, a 225 is about as wide as you should run on those, the rear 8.5" can run up to about a 265, although TBH a 255 would be as wide as I'd run on an 8.5".
If you want to flush the wheels out for a nicer look some quality wheel spacers would be an option. B2 Autodesigns is a good choice, Charles will give you a nice deal.

 

I was wanting to do 2454518 f and 2754018 r. Just wanted to see if it has been done and what it looks like. I guess knowone has done it

 

It's been done, don't have pics but keep in mind running a wide tire on a narrow rim will cause the sidewall to roll under heavy cornering which really isn't a great plan. A stretch mount is better as it keeps the sidewall more stable, within reason of course.
Here's a good link, scroll down to "An Ideal Rim Width Calculator" for a good tool for fitment.
And right below that calculator is a good read~"Too Wide or Too Narrow, does it make a difference"
Car Bibles : The Wheel and Tyre Bible