If the guys from Nissan tested the exact same car as Sportauto, Autocar and Carmagzine did and then did 7.26 min then the time would be more trustworthy. Im certain that the car Nissan used had more power and another suspension setup than the version being sold on the streets.
If the car Nissan used had more power, then why did the GT-R tested by TopGear against a 997TT end up being slower than the Porsche during autobahn roll-on tests? That was one of the Nurburgring test mules. It seems to me that if Nissan really wanted to crush Porsche, they'd use a mule that was noticeably faster than the Porsche, not slower.
As far as suspension, you can't be certain either. What we do know is that the GT-R (unlike many track-oriented street cars) comes delivered from the factory to the dealer with a track alignment. Some of the mags have noticed fairly rapid (and uneven) tire wear as a result. And the suspension is pretty hard. But then it would need to be to keep 1740kg's in check around the Nurburgring.
Porsche's suspicion has been about the tires used, not the power. Nissan responded and invoked the name of Sumitomo Rubber Industries, which has in their possession the Dunlops that set the 7:29 time. So now two companies are (apparently) in collusion to "deceive" the general public*. Meanwhile, Porsche has not disclosed anything about the Turbo that set a 7:38 at the hands of an unnamed engineer.
*IMO, it is the general public's inability (and sometimes downright refusal) to read between the lines and examine the context of the GT-R's lap time which is the crux of the issue. Not any cheating done by Nissan, because we've seen the GT-R being fast just about everywhere else. Nissan released details of their testing procedure, the date of the test, track conditions, telemetry showing throttle position, vehicle speed, steering angles, and have been fully transparent in acknowledging that the time came at the end of hundreds of laps and many different test sessions.
BTW, there is only one legit time for the ZR1 thus far. The faster time noted on various websites has been found to be a hoax.
ZR1 7:22.4 lap time: Re. "Lords of the Ring" - RoadAndTrack Forums
Not that the ZR1 with a racing driver and hundreds of laps could not pull that time.