I too had a similar issue, however my experience was quite the opposite. It was taken care of thoroughly and rapidly by the dealership and Nissan Australia. They both exceeded my expectations. So well in fact I can barely remember it. I wasn't running a cobb or any miracle oil, and after reading of your experience don't think I ever will. Were you running the cobb with the TCU option as well as the ECU?
If the tcu was not configured to suit your tune & mods it will lunch the cog swapper and, I am sorry to say Nissan Aust. would be right to politely knock back the claim. Upon further investigation of the vocal cases in the US it seems that the majority of the cars were also once running tunes, often ECU only. (you may need to use goggle cache to find them as they seem to be otherwise deleted from forums & blogs) An early sign of this could be a "not so snappy" change into even gears. The TCU controls the ECU to turn the power almost off during the shiftand only turns it back on after the clutch is bound again. If the power comes back on too soon or didn't turn down low enough it WILL physically destroy boxes.
Keep an eye out for the lame shift!
Really we cant expect any manufacturer to back a third parties dodgy firmware / software. Out of curiosity, what warrantee does Cobb or others offer on the performance of their product? Choose the performance products that offer the backing! If their warrantee is short or limited, we should ask why? We want confidence and if the manufacturer of performance stuff has no confidence in their products I sure won't have confidence.
Enjoy your car, they are awesome.