I don't recall myself, but Wikipedia says the Atom did its time on a moist track. Given that plus pagani's use of nearly slick tires, I think you could say they're probably at least even, and maybe the atom is faster.
the interesting thing is the 12c is already the 4th fastest time 2.4 seconds off the top time of the huayra, so logically the P1 will be above that. The only question is by how much, but I think 2.4 seconds is not out of reach by any means.