Thought I'd share my impressions.
Car: 2004 G35C with 18 x 8.5 (as measured from flange-to-flange)
Tires: 275/40/18 Pirelli PZero Nero Mud & Snow
That's 0.07% smaller than the OEM rear 245/45/18 Pilot
and 2.65% larger than the OEM front 225/45/18 Pilot.

Fit: In the rear there is plenty of room, as seen by folks with 10.5 wheels and 285 or 295 width tires. In the front they clear at full lock and compression. The tightest fit is at the knuckle (connects the upper and lower control arms): 3/8" clearance (above tire in this pic).

Handling: This was my main reason for upgrading to all 275s. The handling is much more neutral, but there is still a hint of understeer at the limit (as opposed to the back coming around). A bit of throttle and it does a nice drift (autocross stuff here, nothing nutty). The limits of grip in the dry are much higher than the stock 225/245 pilot combination, especially with DCS enabled (I always shut it off for autocross, etc.). In the wet it feels like it handles just as well as the OEM size Pilots.

Braking: No measurements, but 275 versus 225 in the front feels like there is significantly more braking in the dry. In the wet, again, I can't discern a difference from the OME Pilots.

Acceleration: With RT Cats, Ztube, and K&N panel (not much more power than stock), I can't spin the tires in a straight line (automatic, even power braking with DCS off) on decent pavement. Accelerating in a turn the back will come around easier than with the OEM staggered Pilots, my guess is because the front has more grip and resists understeer, allowing easier power-on oversteer.

Looks: I think the car looks a bit more aggressive with 275's on both ends, but I know some folks prefer the staggered look. I'm happy to give up the stagger for the added grip up front and the ability to rotate the tires as they wear. To do this I had to remove the 12mm studs on the front hubs so that the rear wheels will mount up front. The factory must have put them there to stop people from rotating the 245's to the front and the 225's to the rear (the front wheel has a hole to accommodate the studs). Some pics:



Downsides? I haven't come across any yet (1000 miles or so): no DSC or ABS issues, no check engine lights due to the change in front diameter, and no discernable loss in wet traction.

04 G35C 5AT DG/G Performance wheel/tire, no sunroof

 

Thanks for the counterpoint Lou. In particular, I should clarify that I'm providing these observations as datapoints for others to draw their own conclusions.

I should have noted that the 275/40/18 tire I used has a 10.8" section width as measured on a 9" wide wheel. As Lou said, this tire is recommended for wheels 9 to 11 inches wide. That said, I believe there is some margin for error in this specification. I've run wider tires than recommended on several cars (Eclipse, Audi, and WRX) and it is a common practice in autocrossing. I've found that as long as the tire does not crown (bulge in the center of the tread), I will see even wear and expected durability. Different 275 tires vary in actual measured width (e.g. Kumho MX is about a 1/4 inch wider in section width), so use your own judgement. Again, this is just a datapoint, draw your own conclusions. Other datapoints are the folks on the SCCA Solo 2 National Tour running street and race compound 275's on the factory 350Z wheels (stated to be 8" wide). In the picture below (I see the links broke above), you can see the sidewall does bulge more than on the stock 245/45/18, but in my opinion I don't think it is significant.


I may acquire a full-size spare, if I do, I'll shoot pictures of the wheel width. It measured closer to 8.5" in width than 8", as measured by placing a ruler over the wheel and marking the distance from the inside flange to the outside flange.

Some other driving observations:
-- turn-in is a bit sharper with the wider tire up front.
-- turning effort is slightly increased, but not as bad as with wheels that alter the scrub radius significantly.
-- there's a bit more rumble felt in the steering wheel from the wider tire and it is more prone to follow grooves in the road.
-- at the limit the DCS continues to function, as does the ABS, with no discernable loss in performance. I usually shut off DCS when exploring the limits since it is a bit too conservative; more like the normal stability control mode in a C5 Corvette versus the competition mode--where a moderate degree of tire slip angle is allowed.

To further clarify, I am not endorsing that everyone run out and make this change. I am just adding datapoints and noting that I'm running this wheel and tire combination while sharing my observations. Cheers, and if anyone else is running larger tires, please share your observations.



04 G35C 5AT DG/G Performance wheel/tire, no sunroof

 

Just a follow-up datapoint: It bugged me that I might be passing bad info (Internet parrot), so I measured my wheels and shot some pictures showing they were only 8 inches wide. The tire shop told me they measured to be more like 8.5 wheels, so either they were in error or they measured from different points.

They are 8" wide from the inside of the outer flange to the inside of the inner flange. According to a wheel designer at Enkei , that method of measuring is the industry standard (see link).

Looking at the attached pics you can see the measured width and the size designation on the inside of the wheel.


Sorry for any confusion I may have caused.

Another datapoint. I now have ~8000 miles on the 275/40/18 Pirellis and have had no ECU warnings, non rubbing front or back, even wear across the tread, the DCS seems to work fine on slippery surfaces, and the ABS seems to work fine too. I really do like the added grip all around and the more neutral balance that I gained from adding 275's to the front and rear. As an avid autocrosser (not in the G35) I can appreciate the increased turn-in response, reduction in understeer, and predictable neutral handling (not tail happy or plowing). Again, I'm not recommending that anyone eliminate the OEM stagger (225/245), just pointing out what I'm running and that I've had good results.

 

Here you go:

 

Thanks, already have one.