One of the selling points of a synthetic sleeping bag is that it retains its thermal properties even when wet. However, one of the most miserable nights I have ever had was in a wet synthetic sleeping bag in a rainstorm.
Would there be a percentage reduction of the temperature rating or another way of predicting how much insulation it will provide?
Really its all about the loft. So if you have a specific bag you're wondering about, next time you launder it look at it when its wet and see how much loft remains. I know my winter bag loses about 40% of its loft when wet, which isn't bad, especially because it dries quickly even if it relatively humid. There are many different types of synthetic insulation, different types of face fabrics that can affect the loft. There are too many variables to give a flat number. I've heard people say that synthetic base layers are 40% effective when totally wet, but I don't know how that carries over.
Really the best thing to do is take your dry sleeping bag, and fluff it out until its fully lofted. Take a ruler and measure 3 points on the sleeping bag to see how much loft you have and average them. Launder the bag, and measure the same 3 points while the bag is wet and average them. Take the averages and you can calculate the percentage difference.
i = initial measurement average
w = wet measurement average
(i-w)/i
This will give a pretty good idea of how much insulation your bag losses when its wet.
Synthetic will most certainly be more insulating when its wet than a down bag. Down is literally useless when wet. A wet synthetic bag, I'd still sleep in if I had to, a wet down bag I wouldn't even bother(luckily never had this happen). If weight isn't a huge concern, you could try adding a wool blanket. Wool is better than synthetic at keeping its insulation when wet. Wet wool barely looses any insulating properties even when completely saturated. It dries slower than synthetic, but synthetic is generally said to dry faster. But if you're in a rain storm and can't keep dry then wool sounds like it would have been a good option.
I think this is one of the biggest myths in outdoor gear. No insulation is "warm even when wet", the real difference is whether it's life threatening or just really unpleasant when wet.
The useful difference is when the garment/bag is just slightly damp. (i.e. if you get somewhere dry, wring the water out of the garment and does it then provide any benefit?)
The answer for synthetics is mostly yes, but IMHO that's a long way from 'warm even when wet'. Your case is a something we all learn the hard way, being dry is warm, being wet is not. Doesn't matter what gear you have; you have to get in a dry environment to allow whatever insulation value remains in a damp garment to be useful.
