From what I understand, the 500hp Atom will not be built at VIR - that's a separate deal in England... the VIR facility will only feature assembly of the Atom 3, by a company called TMI AutoTech (which took over production from Brammo in Oregon.)
I have ridden in a 300hp supercharged Ecotec powered Atom at VIR and it was fast as stink. Almost "sportbike fast" in a straight line, but much more comfortable. We hopped out of that and into a C6 Z06, which felt slow by comparison! I can't even imagine an Atom with 500hp - it would definitely need wider/stickier tires.
Here are some pics of Chris Kimmelshue & Randy Pobst playing with that 300hp Atom last week at VIR:http://www.get-fast.net/gallery/td308vir/index4.html
Wish I could afford one,