This was a tricky one for sure, much harder than beginner full lap or advanced full lap in the wet.
The key here is to maximize the straights - sometimes this means getting a little lucky and not passing a car when coming out of a corner onto a long fast section. You can lose 5-10mph coming out of a corner if you are trying to pass someone, and that can lead to 2-4 seconds depending on the section. Even by yourself, this matters a lot. If you get 1 corner "right" it should be the the one before the 2km straight. Losing just 2km entering the long straight can mean 0.5s slower on the entire lap.
This probably took me about 8 tries, including 4 laps in the 7:05.X before putting down a 7:04-flat. I had been playing all day (finishing off licenses) so I was pretty strung out and was making stupid mistakes. I am convinced a 6:59 exists out there without cheating.
Also, note about passing - although the cars are supposed to pull to the right, in the SLS you come up too quickly so they can't see you. There are cases where passing on the right instead of the left might be faster.
If you are struggling, go back to the first 4 tests where they break out the course. You should be able to get gold by a full second on each of those tests. The full lap isn't quite the same pace, but there isn't a very large margin of error. This test is much easier than the GT4 'ring challenge in the SLR.