Porn is acting. It's not all gonzo which, by the way, is also acting. And to claim wrestling is "good acting training" is a bit of a stretch. The Rock had one of The Best characters in wrestling but he's only a mid actor at best. Bautista, OTOH, never had a particularly compelling wrestling persona but he's an amazing actor.
I don't see why it matters if they play whores or prostitutes since they're still portraying another character. When Randy Savage portrayed a wrestler in Spider-Man, he was still acting. He "crossed over." When Ray Allen portrayed Jesus Shuttlesworth in He Got Game, he was acting. He "crossed over."
When Bella Donna appeared in Inherent Vice, she showed no nudity. She played a "porn star," IIRC, but she could've easily been playing a secretary or librarian for all her character did. Traci Lords also showed no nudity in Blade nor was she a sex worker even though it was rated R. They both "crossed over" even by your standards.
I didn't know that about Jamie Gillis or Harrie Reems. I do know there was a Black actor, John Bailey, who appeared in Good Times, M*A*S*H, Happy Days and Kentucky Fried Movie who appeared in a porn called New Wave Hookers with Jamie Gillis. He crossed over the other way.
So, yes, a porn star "cross over" is the same as a wrestler, musician, professional athlete or anyone famous in another field doing a film.
Edit: I just looked up Jamie Gillis. He did not start out in mainstream film. His first film is Deep Throat Part II in 1974. He does only porn films until he stars in Nighthawks with Stallone in 1981. From my quick research, Reems only ever did porn and never crossed over.