Guys, we should be careful with numbers because nobody is actually using qualified and/or certified equipment for testing.
The best real life test is back to back with all cars at the same time on the same track. I know this is difficult but this is the only way to get an accurate comparison.
I also prefer to use the speedo speed (not the real one) as a reference point because the speedo accuracy on most 997 Turbos is similar, usually it is 5-12 kph off the real speed, depending on the speed level.
My 997 Turbo needed 25 seconds from 0-250 kph as far as I remember. If the tuned RUF 997 Turbo with claimed 550 HP does the same 0-250 kph performance in aprox. 8 seconds less, this is pretty impressive.