Most accurate way to put it is that *at least* some were delusions. Maybe all but we can't know for sure the way it played out. Like when he opened the closet in the other apartment and the bodies weren't there, or Paul Allen being in London despite his memory of killing him. That he would have gotten rid of the evidence so smoothly without recollection seems too farfetched to me.
Discussion
Yeah, I agree we can't know for sure. But I think Paul Allen being seen in London can be explained by maybe it wasn't actually Paul Allen - mistaken identities were a common theme among Bateman's peers. Even Paul Allen mistook Bateman as Marcus Halberstram the whole night they were together.
As for the bodies in the closet, I always thought the lady showing the apartment acted very strangely - asking Bateman to leave so there wouldn't be any trouble - as if all she cared about was selling the apartment, and maybe cleaned up the bodies herself so that they wouldn't get in the way of her sale.
Ok with Paul Allen, but the lady was right to be acting like that, in her eyes there was a total stranger there that had no reason to be in that apartment, as he wasn't her 2 o'clock appointment.