Porsche has released the on-board footage of one of Neel Jani’s laps of Spa-Francorchamps in the Porsche 919 Hybrid ‘Evo’ last month. The one in the video is two seconds slower than the 1:41.770 he set later on – for that lap the team took the camera out of the car to save uneccesary weight!
First of all, I believe the quote was 1324kg of downforce at 320kph. Hardly enough to "drive upside down".
At 160kph it would have 1/4 of that, i.e. 340kg of downforce. Hardly too much a difference to the 230kg of downforce the koenigsegg would have. But if it were cup2 tires, good for the Vulcan. I honestly was a bit disappointed at the laptimes from both the Topgear and the GT.