I would think that if the GTR and CGT were driven with the same tires, on the same track, and at the same time, the CGT has to be faster given an expert driver.  Of course, if the driver is not an expert in driving the CGT, the gap will be closer.

Take note that the Nurburging has probably gotten a bit faster since Sport Auto tested the CGT some time ago.

I don't know but CGT mainteance is on a different level than even the GTR.  Engine oil changes are like $1k, and clutch change could be over $20k.  But I've had the CGT at like 13k miles and so far the clutch still checks out okay, so it depends on how you drive it.