Here is a link to a ukc article on meltdown...the vid is pretty short, more of a letdown than a meltdown, but in what's shown, it sure looks a lot more like a face climb than a slab climb in the sense that I've normally understood the term "slab". Not that it isn't unbelievably hard, just that he's crimping and edging and sidepulling... www.ukclimbing.com/news/item.php?id=67243
Possibly still Tokio Muroi's Banshousha in Ogawayama in Japan? - repeated several times, including by Nalle, so if he thinks his new ones are harder they probably are. Banshousha is 8Bish so V12/13. There is also one Muroi did next to it that is harder and still unrepeated I think.
I don't believe Font has any slabs harder than 8B (have not looked carefully though) but those grades are solid (I saw slabmeister Philip Le Denmat fall off a 4c slab once at Font - made my whole trip!)