It's not so much that I'm skeptical as my carrier keeps its monthly rates down by not giving a phone subsidy and I'm not about to shell out $720 for a 5S to test on.
Might get a Nexus of some kind at some point though.
I'd be far more concerned about compatibility with various advanced subtitling tricks (karaoke, signs typeset to blend into the scenery, etc.) than compatibility with 10 bit. Last I heard the iPhone 5 would sometimes choke on those, not sure about the 5S.