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.