Dude it would be off on the road.
In fact help me here!
When I've driven a GTR on road I set the three switches too:-
Gearbox: R
Suspension: Comfort
Traction: R
I found in R on traction every single roundabout the GTR waggled its tail with quite a good slip angle but it was almost as if it did it on purpose with the differentials just to make me look like a driving god which was no issue. A little like the 911's sport mode which detects what the slip and steering angles are and if your in control it leaves you to it, infact I love the PSM system on the 911 it really is only there for when you get it wrong otherwise it will let you play without interference.
What are the gearbox modes and traction modes doing exactly? Obviously the suspension one is self explanatory, comfort hard, normal harsh and race back breaking.
Should really move that post and this one to the GTR thread
I also drove in R/N/R most of the time because it makes you feel amazing
The Gearbox switch does a couple of things.
In R mode it holds the gears longer in automatic, shifts slightly quicker than Normal and when manual/paddle shifting it won't shift to the next gear at redline, it will just bounce off the limiter if you let it, in Normal mode but manual shifting it will auto shift up still at redline, which is really confusing if you pull the next gear at the same time!
As you say, the Suspension switch is self explanitory, C is good for launching, N on the 2012+ is fine for everyday use and R is back breaking mode.
The traction switch which is really called VDC - Vehicle Dynamic Control, does a few things but I don't have any hard numbers.
In normal mode it pretty much won't let you get the tail out, it has control over the engine power, torque split, uses the brakes to keep you pointing straight and will burn through pads pretty quick.
Put it in R and it then will let you use launch control, sends more power to the rear, allows good amount of slip and doesn't cut power so much. It will still wear out your brake pads when you drive really aggressively in this mode but it keeps all the sensors running to help you out.
Hold the switch down until it goes orange and that is VDC off, only ABS will be active (so good luck!), apparently the clutch pack (FWD clutch) that sends power to the front can wear faster on track with this mode and Nissan say it is only for getting the car out of mud and snow if you are stuck.