I decided to run my own, more appropriate tests after seeing this thread for the sake of those asking for it rather than my own amusement. Note now that my test is not supposed to reflect any real life performance (there's no reason to take my results as a real world indicator or vice versa, let's keep it strictly GT5).
Brief
My test was also ran on Laguna Seca, mainly to use for comparison to the 'stock' lap times set by the OP in this test but I decided to run the ZR1 and Spec V versions of the Corvette and GTR respectively as in GT5 they are much closer on price and also represent the top spec version of their model ranges (available in GT5). Also note I would consider myself a reasonable average driver, using a G25 (cockpit view) with good car control though not the fastest lap times. I ran each car for a total of 5 or so laps a piece, and will take their best laps as the results but will also include discussion and conclusions which I can take from my short, though reasonably thorough test of these 2 cars on Laguna Seca. Only modification to the cars, was aftermarket wheels as my ZR1 already had some fitted and so the SpecV for the same wheels for consistency with both cars running Sports Hard compound tyres. Edit: It's also important to note I used paddles for the SpecV and full clutch and H pattern for the ZR1 in the interest of realism.
Results
Nissan GT-R SpecV '09: 1:32.915
Chevrolet Corvette ZR1 (C6) '09: 1:33.773
Conclusion
The differences are of course going to be obvious, we all know the ZR1 high power output will help it fight back against the SpecV on circuits with more high speed flat out sections (Nordschleife) but whether this would be enough to gain back what is lost from the difficulty in getting that power down effectively (at my driver level) I would not know without further testing.
The ZR1, I find, is actually not a particularly difficult car to drive in GT though that is certainly not because there is a lack of oversteer. There's plenty of it, particularly coming out of the lower speed (2nd gear for the ZR1) turns of Laguna. It deals well generally well with the circuit however, turn in is relatively crisp considering and once the traction is there the tower of V8 power will pull you much better then the GT-R can manage however the main weakness is simply putting it all down coming out of the turns. It was however a more enjoyable experience, though more laps were wasted making mistakes as the ZR1 is nowhere near as forgiving where the GT-R will quickly allow you to resume your lap.
In great contrast the GT-R would fire itself out of the low speed turns with extreme traction, making up for the lack of top end power when compared to the extra 100+ ponies that the ZR1 packs as there aren't enough places on the circuit to really utilise that force. Direction changes felt quicker in the GT-R also and generally the car would react quicker to inputs. The GT-R is not without fault however, it did have a tendency to understeer if to much power was applied to early but this trait is easy to avoid and once you drive the correct lines for such AWD power the lap times quickly tumble whereas my speed increased more progressively in the ZR1, I made a big jump towards the end of my GT-R session due to needing to adjust after being in such a different handling car. Also of note is the GT-Rs double clutch shifting made it easier to concentrate on your lines but I feel that this difference alone would not make up the difference between lap times, my clutched lap with the ZR1 felt solid with no mis-shifts.
Ultimately I found the ZR1 was more exciting, but there's no real argument that in most peoples hands the GT-R will be more consistent and ultimately faster for performance driving. However if we're talking bang for buck the extra premium on the SpecV compared to the '07 GT-R is simply not worth it unless you have cash to burn.
Which would I buy? The GT-R SpecV if I wanted to win, the ZR1 if I wanted to go for a drive solo or the '07 GT-R if I was tight on cash (you could easily modifiy it up with the spare cash to be a quicker machine at the same price as the others stock).
Could the ZR1 be faster on different tracks or with a better driver? Probably yes on different tracks but here at Laguna I don't think, though I may be wrong, that many drivers would be quicker in the ZR1.
I would note however for tuning purposes, as a driver who prefers to keep most road cars to that, I have found much enjoyment from a SpecV at around 600bhp (with all the relevant handling and weight loss treatment) on SS tyres as a road legal time attack machine.