Gore tex/breathable waders are fine in cold water as long as you layer properly underneath them. I stopped wearing neoprene probably about 15-20 years ago and have worn nothing but my breathables since then. I'm a big steelhead angler so I have worn my breathables here in Michigan during some of the coldest Jan/Feb weather you will find in the Great Lakes area.
Normally what I do is I wear a pair of wicking under wader pant as a base layer, then fleece under wader pants, then my breathable waders. That is usually just fine most the winter when the water temp is 40-45f. On those REALLY cold trips when the water temp gets down to 36-38 then sometimes I'll do another pair of long johns between the wicking layer and the fleece layer. Only thing that ever gets cold is your feet so you have to step out of the water about once or twice an hour for a few mins, but that's just because stocking foot waders don't have that cushion of air around the feet like bootfoot waders.
Thank you VERY MUCH for that information. That was precisely what I needed to know. I just ordered a pair of Simms G4 Stockingfoot waders for a good price - $639.
I have not fly fished in over ten years - we lived in Chicago for 11 years and moved back to sunny New Mexico about three years ago, so I'm out of touch with the latest technology. But one thing is a constant - I have always hated putting on my neoprene waders, but that hopefully, is now a thing of the past.
I have all the fleece undergarments I think I will need to stay warm. Feet, just as you noted, are another thing. I'm hoping that I will be able to wear a pair of heavy socks under the waders. I say this because I had to buy the L(12-13) sized waders which are slightly too large for me. But, I have large feet - size 12, and the correct inseam waders would only accomodate a size 9-11 foot. So, this may be a blessing in disguise if I can slip those heavy socks on and get them into the oversized waders. We'll see how this all plays out.
Getting out of the water every once in a while is not that big of a deal. So if I have to do that that will be fine.
Again, thank you very much for your in-depth response to my query.
Pizza's on the way!