Provisions against simulated child pornography were found to be unconstitutional in Ashcroft v. Free Speech Coalition [0] in 2002.
From wiki [1]:
> Referring to [New York v. Ferber, 1982: child pornography is not protected speech], the court stated that "the CPPA prohibits speech that records no crime and creates no victims by its production. Virtual child pornography is not 'intrinsically related' to the sexual abuse of children".
IANAL, but following that logic alone, the degree of realism doesn't seem to be relevant to the legal precedent insofar as photorealistic imagery would still "record no crime" nor "create victims by its production." As to whether it's dangerous for such material to exist because it would create plausible deniability for the production of actual photography while claiming it's simulated...I guess that would be a different matter.
[0]: https://en.wikipedia.org/wiki/Ashcroft_v._Free_Speech_Coalit...
[1]: https://en.wikipedia.org/wiki/Child_pornography_laws_in_the_...