As long as the hills are rolling the extra weight isnt that bad, because momentum helps them up again for quite a bit. This is the case for most of PBP. I think the Kernow and South West would be a very hard ride, even for the sub-60 hour PBP velomobilists. I'll offer 100 pounds for the first velomobile to finish it.
AFAIK Wim Schermer has never done an audax, let alone PBP, so where his name comes from in this example, I do not know.
I think LWaB is right that most velomobiles on PBP are ridden by fast riders, and the slower ones get a lot more sleep than the average upright.
Tigerr: I tend to lose considerably less speed on long not-that-hilly rides than uprights, but that might just be me. After all the hilly bits in Mille Miglia I was able to average 23km/h on the last flat 120km stage. I was asked how I could go that fast after such a long time by a fellow Dutch audaxer (PBP, LEL) with a Raptobike. Well... it was flat...? I can do 25km/h on a flat bit without wind, even when knackered.