I do not have experience with an UQ but I can speak a little to the physics. The phenomenon you are referring to with rising warm vapor is convection. It is the reason the moisture in your breath condenses on the underside of your tarp. The force that causes convection is the result of the slightly greater weight to volume of the cold air to the warm air. I am sure you are aware of this but my point is that it is a relatively weak force that is temperature driven (not moisture driven) and is easily disrupted by friction, such as the air moving through a fabric experiences. Convection can only happen if there is cooler air readily available to displace the warm air which will not be the case within your insulation system where trapped air has already been warmed by radiating body heat. Warm moist air next to the skin on your back will still tend to be displaced by cooler air in the upward direction but likey not fast enough to make up for the diffusion outward towards less moist air. That all depends on your weather conditions of course - temp, rh, wind. Anyway, the moisture in the trapped air will tend to move in the direction of lower concentration which for your back is going to be directly through the UQ.
I understand 2QZQ makes a ripstop UQ protector to allow moisture to escape the shelter, but by the time the moisture reaches the protector whether it be sil or ripstop it has already passed through your UQ, most likely meeting the dew point before it arrived at the protector. I'm not saying you can't see any benefit from the breathable protector as it does let moisture that your down didn't absorb escape, but in my opinion the best solution is a close to skin vapor barrier as has been suggested.
I can't help with recommendations for specific conditions as my experience is lacking but that is hopefully some helpful explanation of the physics.