How will a hybrid water heater perform in less than optimal conditions?
For your “with ducting”, it should not be an issue at all, since you have met the full requirements with ducting.
For your “Without ducting”, in general you have met the condition, as the 300 cf room has an opening to a larger space, which meets the requirement.  
In your “worst-case” scenario, it would perform just like a normal electric hot water heater with the same performance characteristics.
I have a hybrid which I installed several months ago in a true “worst-case” scenario.  It is installed in a small utility closet, 4x3, which is in the laundry room, 8x10.  I just leave the door open.  I leave the WH locked in the heat pump mode, as I see no reason to rush to heat the water in the tank back to full capacity.   If I have guests or for some other high demand days, I could manually switch it to hybrid or high demand for those occasions.