Something's not right here. If Q55 has plenty of gain, then there's no reason for Q850 to run hot. Are you absolutely sure Q55 checks out OK? I wouldn't necessarily trust the measurement without knowing more about the tester and setup.
I assume you've been using a scope to measure ripple, so you would have noticed if there's any high frequency oscillation that can mess things up. If Q55 is definitely good, then I'd focus on Q850 now and figure out where all the currents are going and how big to account for it getting so hot.
Ed
