My setup, in all honesty, is not a million miles away from default. I know some people are fearful of setting the radiator to the minimum level but even over the course of this circuit, it doesn't bother the Ferrari (3 or 4 engine damage max.) or the BMW (20 engine damage max.), which don't really harm the power/top-speed. Of course, running 2 laps will start to see your engine blow.
Brake ducts and getting the right starting tyre pressures are the fundamental key, for me, in nailing the perfect lap. Close the brake ducts too far and you'll push the tyre pressures through the roof - on 0% brake ducts I was seeing in excess of 2-bar at the peak. Leave the ducts too open and both your brakes will suffer, as will your tyre pressures. The setting I have it on is slightly compromised for braking performance (ie, they get cold by the end of Döttinger Höhe) but I peak at just over 1.8 bar.
I gave it another 5 or 6 laps yesterday and although I was consistently faster in S1 and S3, I was well off my S2 time, leading to slower laps overall. I think that middle sector was one of those once-in-a-lifetime efforts, that sadly was book-ended by rubbish sectors (by comparison)
EDIT : Also, without Puffpirat setting his blistering lap, I would not have been spurred on to do this lap. Even after I went (fractionally) faster than him, I knew an 8m03 lap was within my reach. Of course, having done that I realised I was capable of an 8m02 lap. In theory
. I have yet to run a single perfect lap in time-trial, not once have I ever sat back and said to myself "there is no room for improvement in that lap, at all". I've come close but there is always something left on the table.
It really wouldn't surprise me to see somebody with the perfect setup and THE perfect lap, do sub. 8m. After all, with ~180 corners in the lap, it only needs somebody to improve on my lap by less than 0.02s in every corner to do it. Two hundredths is nothing.