In Extremis from Norgannon-KR just killed Ragnaros 10man Heroic.
They're a 25man killed usually, they killed all heroic bosses on 25man but just looted 2 397 items and 1 head token from Ragnaros.
Wowprogress records their kill as 10man as well.
Grats to them for being the first guild to kill him on 10man after a 25man guild did it!
EDIT: In Extremis got ALL their guild first achievemnts in 4.0 in 10man. They also killed Baleroc HC, Majordomo HC and Ragnaros HC and all normal Firelands bosses in 10man. You decide if they're a 10 or 25man guild.
EDIT 2: I've scrolled through their armory and they've only killed bosses on 10man the last 2 weeks. Can't scroll back more than that. Once again, you decide if they're a 10 man guild or a 25man guild.