Ferrari 550 Maranello lap time at Nürburgring Nordschleife

At Vairano Lambo beat Ferrari:
https://fastestlaps.com/comparisons/9o4d2e6nep1b

Anyway, at the Nordschleife with proper testing both had to be faster than 8 minutes.

I'm not sure about that. I mean, both cars don't have a reputation for great track performance. I think I remember Mathias Lauda saying the 575 was rather sluggish.

Sport Auto laptimes usually not very good due to the limited time on the track, and the fear of not damaging very expensive car, so better times very possible.
However if this cars had Porsche badge, at least 10 seconds could be cutted:)
As for motorsport, if I'm not mistaken, Prodrive 550 had won FIA GT Championship. Diablo can't boast of that.

Good point. But I'd say considering the time these cars came out, not much of a difference could have been done. Manufacturers weren't so good at compensating for their weight, and similar Ferraris or Porsche weren't that much faster.

Sport Auto has always been traditionally tougher on cars that aren’t German, much like how Best Motoring tends to be tougher on non-Japanese cars.
In reality, those cars are often quicker than the results suggest.