Couple things from my experience. I don't want to seem like I'm refuting other's advice; everything is HIGHLY RELATIVE and at the end of the day, if a Bivy didn't work it wouldn't exist.
#1: When I sleep in a tent with a single wall, I tend to have condensation issues in midwinter. We'll wake up to icicles on our tent's inner wall. However, my other shelter is a Henessey Hammock, which has a ludicrous amount of ventilation. It's obviously colder than the tent, but dry as a bone save for a spot of ice on the netting directly above my face. What this tells me is that the only mitigating factors to condensation, given the nature of materials as they are now, are Temperature and Ventilation.
If you increase temperature, similarly, you must increase ventilation or condensation will form.
#2: I have soaked synthetics completely through and then slept in them. One occasion, a rainfly ripped during a storm, leaving me with a flood of water in my hammock. I shivered, but I was already pushing the bag's limits and was grateful for the synthetic insulation. The same goes for clothing; any experienced backpacker will tell you just how important a fleece is in wet weather.
I have soaked a 20º synthetic bag from ocean spray on the coast of Maine in early april, and I've soaked the same bag in snowmelt in New Hampshire's Mount Moosilauke in February, so I've got a pretty good idea of the expected warmth retention.
I'm a little surprised the idea is up for discussion; I thought the warmth when wet concept was not only ubiquitous knowledge, but pretty undisputed. I can be enlightened, though.