I acknowledged Juha Bach's power, but I have to disagree with the notion that Bach Superseded Yammaji in power. The only reason that Bach could kill Yammaji was because he sealed Yamma's Bankai.
It's the same as Byakuya dieing. The sternRitter who got/killed Byakuya would not have had a chance if he hadn't first sealed Byakuya's Bankai. In the same way Juha Bach would not have been able to kill Yammaji if he had not first sealed away his Bankai.
As for how Yamma would have fared against FGT Ichigo, Transcendant Ichigo, or Transendant Aizen..Of course Ichigo or Aizen would have destroyed Yamma.
Steve, looks like you beat me to it. I completely agree.