That's hard to answer because the advantage is essentially infinitely large. Engines never lose or draw against unassisted humans. Any modern chess engine, if it plays 100 games against any human (even magnus), will have a record of 100 wins, 0 draws, and 0 losses. This is true both in standard chess and Chess960.

I'd expect them to have a larger edge in chess960 because humans can't prep openings like in regular chess.