Never flown the Dash 8Q-400 (even though I used to dispatch it all the time) and I am not aware of any QC kit or full-freighter version. The reason I guess is that it could only be used as a
bulk freighter since the cabin diameter (smaller than the ATR's) wouldn't accept containers. Also, I can testify that as a PAX a/c, it can be a bugger to get "in trim", especially with a lightish load. I believe the extra performance provided by the Q-400's chunky engines is not really a "plus" for cargo ops - where speed is not as important as dispatch reliability.
Cheers
