Here's a copy of my post on that thread:
My ultra-light Western Mountaineering bag is a masterpiece: super-quality down, excellent fabric, beautiful cut. It stuffs unbelievably small for the warmth. I've slept in it (really slept, not just dozed & froze) in a bivy sac at 11,000 ft. in the low 20s with snow and wind.
A good river combo is an ultralight bag (the Big Agnes bags are also excellent), a bivy sac with netting and a bit of headroom, and an ultralight tarp (I've got SILTarps and a SILShelter from Integral Designs that fit in a coffee mug. I've camped out with them on South Island NZ in bloody downpours and stayed reasonably dry).
The key difference with a synthetic bag is that if it does get soaked, you can press out the water and sleep in it. It'll be damp at first, and might stay damp underneath, but for the most part it'll dry out with body heat. Which can save your life. A wet down bag is impossible to dry without considerable heat and fluffing. If you get down, get the best possible lightweight drybag for it.
Integral Designs used to have some super lightweight synthetic bags as well. I got a roomy but light Primaloft model that works to mid 30s, with a liner bag that takes it into the 20s (for me, anyhow).
A few added notes: if you'll be running western US desert rivers, with dry conditions on the banks and few extended storms, down offers more comfort for the weight. You need to dry it out each day before stuffing it.
But if you're running rivers in deep shady gorges in areas of high rainfall and humidity (west coast NZ, British Columbia or SE Alaska, NE coast of US, or the Tropics) then a high-end synthetic bag is a better choice, since it will mostly dry with body heat.