I don't think the nosedive was necessarily bias. That's precisely the behavior we've seen before of similarly accelerating cars, when one has a (sequential) manual vs the other with a DCT. The SLS is stealing back precious tenths of seconds on each gearshift compared to the LFA. The SLS should be faster off the line with its launch control; the prototype LFA's don't have it, but it's said to be on the production ones.
Autocar have previously commented on how they perform these types of comparos:
"It's also worth knowing how we go about filming these drag races. Although in the finished vid you see one single race, we're a tiny bit more scientific about it. In this case, before we filmed anything both Matt and I had a go at figuring both cars, and without the weight of Andy the camera man [contrary to their official road test procedures]
. Then when we were satisfied we'd got the best from both we filmed the race to fit the numbers."
Porsche GT2 v Corvette ZR1 - how the film was made - Tester
If there's anything to take away from this video, it's that...the British car is the fastest.