Actually I think you're wrong there martz, sure 'ring times are great for marketing but I wouldn't say it's an even playing field for comparing cars. The main reason why is it's simply too long and a public road.
Rarely, if ever, does a car have the 'ring to itself for the entire lap, there's usually traffic somewhere unless they've booked out the whole track. Now because the track is so long there's usually large variations in track conditions such as weather. And of course there is always a different driver at the helm.
so while I would accept the times, I wouldn't necessarily say it is the 'fastest lap' a particular car can do.