I always thought that the reason danzo mentioned him as "someone who can control a bijuu" is because he was under tobi's (or madara) control. if tobi or madara could control kyuubi he could also control sanbi.
Eeeerg that is something I've never once considered.
So you are saying that people were aware that Yagura was good at controlling his biju because he perhaps had been seen transforming like Bee.... but you are saying that he only did that while being controlled and never on his own? That is not at all what I thought when reading that, but it could possibly be true.
I interpreted it as Yagura having some sort of natural control over the beast which might have lead him to becoming Mizukage, since Madara/ Tobi likely didn't start controlling him until after he reached that status. If that were the case, then we'd have to assume he could at least partially transform without loosing his mind.
That being said, yagura was also appointed 'representative of the jinchurrinki and biju' by the collective group. Since he obviously wasn't being controlled then, I think its safe to assume they were referring to his prowess pre- mind control.