So I can finally share some experience after a few trips with the 2019 Discovery and the Flying cloud 19 Airstream:
After having towed with and without the Blue Ox Sway Pro bars installed, I can say that at least to my feeling, the car tows better without it. Much smother ride, no sway and natural fell. This weekend was really windy and other trailers were suffering down the highway, but we could go 60 mph with no sway. The only thing is that the ball moves more inside the couplers and you fell it when pulling out or braking.
I am sure this combination can pull a much bigger trailer, as a matter of fact you have to be careful when pulling the 19 because it is easy to go over 70, you don't even notice the trailer behind sometimes. Anyhow, we are afraid that in the disgraceful event of an accident being over the factory limits can become a liability issue, and that is something to be consider. We have been towing over the limits with our Flex for 3 years, so no pointing fingers at all on my comment, just thought for those considering it as we did.
We have a 23 on order and we are not going to use the weight distribution hitch, but we might consider some kind of simple anti sway for additional safety.
We are in a similar position to what you initially posted - we own a 2020 Discovery & want to purchase a 2021 Airstream 25 Flying Cloud. We currently have a 22 Airstream Sport that we towed with an Explorer. With the new car we want to do bigger. Just curious what made you go with the 23 vs 25ft?