If this helps I will give you my experience from this past Saturday. I knew rain was in the forecast, that it could at times be significant and our team might be dispatched for some swiftwater rescue calls. I decided to wear a synthetic t-shirt, nylon pants, wool-blend socks, cotton underwear and a cotton overshirt thinking I would strip off the cotton shirt and don a long-sleeve Nomex t-shirt and my dry suit.

Unfortunately, when we got hit out for our first call, I forgot my Nomex t-shirt and did not have time to put on my dry suit before responding. At one point I had to exit the unit to check a flooded roadway to see if we could safely proceed. I got quite drenched, even under my PFD.
Just as we finished with that call we got dispatched to another call so I could not change. We put the heater on high to try and stay warm and dry out a little. We got a break in running calls and stopped for gas and dinner. I removed the cotton shirt and tried to air dry my remaining clothes while eating, but did develop a chill.

Just as we finished dinner we got another call and this time due to having to enter deep moving water I put on my dry suit over my wet/damp clothes. We ran calls for several additional hours wearing my dry suit the remaining time and was warm the entire time. When we finally got back to the station, I hit the couch for an hour still in my dry suit and was warm. After an hour of not running any more calls, I decided to call it a night and head home. As soon as I removed my dry suit, I got cold.

While a dry suit is thicker than a plastic bag, it is not insulated. Based upon my experience, I think if you had to be wet, it would be better to try and remove the excess water from your clothes and place the plastic bag over your damp clothing.

Pete