I`ve owned both the GT2 and the SL55 (delimited) at the same time and made comparison tests between them on the autobahn.
We started at 200km/h and accelerated to 300km/h.
Result:The GT2 was considerably faster.
No way those figures from Nardo can be correct.
If I remember correctly the time of the GT2 was 40.5sec.
If a E55 does 47sec I guess an SL55 could do 44-47sec.
The drag coefficient of the SL55 is 0.29 compared to 0.38 for the SLR which makes a huge difference at high speeds.
Sport Auto magazine tested a RS-Tuning GT2 with 542PS
which posted a time of 28.5sec from 0-300km/h and a lap time of 1min 10.5sec on a slightly damp Hockenheim circuit.
The car had a different suspension kit as well (Uwe Alzen Motorsport)and Yokohama Advan 048 tires.
Should be interesting to know how fast the Carrera GT is.
The Cd value of the Carrera GT is 0.39.
The goal of the SLR and the Carrera GT is to produce a lot of downforce not straight line performance.