IME...they do ....but to a degree.
Insulation is all about loft....and most syn's have more structure to them over say down when damp.
As to the debate on syn vs down.....I think its all about having the right tool for the job. In most cases, down works sufficiently even with some moisture buildup...though I have been on hunts where 3 days of heavy moisture laden air decreased the loft on my down bag by 50%. I've been on hunts where mice chewed through my bivy and the melting snow soaked about 1/4 of my syn bag [and froze 1/3rd of my pad]...but after 20 min I slept like a baby even wet and was fairly dry by AM.
Thanks for your help. I have read a lot of the down vs synthetic discussions and understand in general. I was just curious if all or most synthetics still perform pretty well in damp/wet conditions. Or if some are considerably better than others? This is mostly coming from most sleeping bag manufacturers having their own "brand" of synthetic insulation. I suspect they are all fairly similar. Just trying to confirm that and determine if there's much difference in wet/damp performance or if the main differences are just physical weight between synthetics. I hope that makes sense and isn't too confusing.
I can't say Ive seen any type of lab testing synthetic vs synthetic warmth retained at saturation, as its always a debate of syn vs down. However there are certain types of synthetic insulation that perform better for longer which means a longer garment life and could possibly translate into what your talking about. One of those is Apex climashield that is usually at the forefront of the conversation. Im sure if you go over BPL you will find more info on this subject as I'm just regurgitating sh I read elsewhere.
I've heard it claimed that Primaloft retains more loft when wet then Climashield, but I have not seen conclusive evidence to back it up. Kudos to primaloft they do list wet and dry CLO values for all their insulations, while Climashield does not.