Here is a (random) list of some hobby-racers with various cars/bikes

http://bridgetogantry.com/index.php?option=com_content&task=section&id=4&Itemid=28

As a rule of thumb I'd say:

10 min.: you're just starting to feel the "flow"
9 min: nice - IMO not achievable for a novice (whatever the car...)
< 9 min: pretty fast (I'm still working on it - though not with a 997 TT )
8 min: sounds like a pro

Of course I have to agree with MKW Mind you of the famous lap Sabine Schmitz did with a Ford transit

http://uk.youtube.com/watch?v=Lg4a1rc4Xjk


I like this remark on the BTG website, especially the reference to the moonphase

"Note: The new calculated power to weight ratio is only as accurate as the data supplied with the claim. The lower the number, the faster the vehicle. Theoretically. The reason the higher numbers can be faster than the lower numbers is driver/tyres/suspension/moonphase."