MKSGR:
P.S.: Bikes stand no chance on a race track (not only NBR)
The corners are the enemies of the bikes
The official record (also with Dähne driving the bike) is almost 20 (!!) years old: 7:49 on a stock Honda VFR (RC 30) - that was back in 1993 (certainly faster than any stock Porsche at that time). Since then there was no officially registered new record lap. It is said that in 2008 a GSX-R 1000 (IDM Superbike = race bike on R-Tyres) did a 7:32, but there is no official record of that lap. I'd say today's fastest stock bikes may achieve a laptime halfway between Dähne's record lap and the said race bike's lap (7:40ish).
Here are some nice Video's (though only BTG, no full laps) showing very fast laps on a 2004 R1 - looks like one needs balls sized like watermelons to do the 'Ring on two wheels at this speed
http://www.slidersguesthouse.com/ringvid.htm
@ Porker: it depends on your car's set-up and on how much you love your car I prefer to avoid the curb/rattlestones now but I'm still not in love with the new line (the former speed / braking point / turn in point is still stored in my brain ).
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public roads: Porsche 987 S Seal/Cocoa, toll road : Porsche 997 GT3 Arctic/Black