Re; waterproof Gortex shells, are a double edged sword, they are less breathable than pertex so can more easily contain/ trap perspiration moisture, reducing the efficiency of down...but even the most breathable shell materials can’t entirely stop the dampening effect of such internal moisture passing across the surface of down.
No maker has offered a "waterproof" goretex shell for about 25 years...
When you think of GTX you normally think of various other competing technology laminates in the same technical performance context.
My words still correctly apply to any pre-existing GTX-WP s/bag...and to the sequential mind -also to any past or present non-GTX
waterproof-breathable-laminate shell construction s/bag...be it Exped, Mountain Hardware, etc
Calling what is commonly referred to as WPB shells (waterproof breathable) "Goretex" is like referring to all wide-body planes as "Boeing 747's." The 747 may have been the first, but it's not the only, just as Goretex may have been the first WPB PTFE laminate, but it's not the only player. Calling all WPB shells on sleeping bags "Goretex" is also ironic, since no sleeping bags shells are made with WPB Gore products any longer.
Also, you're confused if you think Pertex is a monolith fabric that does not include WPB fabrics (which you mistakenly refer to as "Goretex"). Pertex makes somewhere around 9 different shell fabrics, 3 of which are WPB laminates like traditional Goretex.
And, of course, the Goretex Co. makes all sorts of fabrics, some of which are WPB, some of which are not. Also, there are a dizzying variety of proprietary and non-proprietary WPB fabrics anymore. That's why they're given the moniker WPB to refer to them as a whole (not "Goretex").
As to WPB shells on sleeping bags, I got my first actual Goretex shelled bag in 1982 (I got my first Goretex Jacket in 1978). It was a Colorado made Marmot Mountaineering bag that I used for ten years. In 1992 I got a Western Mountaineering bag with an actual Goretex WPB shell that I used until 1998.
What I found about Goretex shells, and what Goretex subsequently found, is that a WPB shells can create a slightly "clammy" feeling in the bag, and that in extreme cold may actually fight against the bags "interior climate" by not allowing moisture to exit rapidly enough overnight, allowing it to freeze in the outer layer of down, thus degrading the bags insulation. Of course, the same can happen with any down bag (and I've had it happen) which is why I would always use a VBL liner in a down bag in extreme cold, whether it has a WPB shell or not.
There are many circumstances where a WPB shell on a down bag makes sense for extreme conditions accompanied by wind/dripping water/frost/condensation. I'd just make sure I use a VBL. I personally wouldn't use a WPB shelled bag in conditions above an arbitrary 10*, and to me, they're at their best below zero.
Back to actual WPB Gore sleeping bag shells, Goretex replaced free of charge my Western Mountaineering bag because the Gore laminate started to delaminate in the hood because of hair oils. Gore must have had enough problems, because they stopped warrantying its WPB laminates on down bags sometime in the 90's. That's what they created GWS (Goretex Windstopper) for. GWS is the only Gore product now offered on sleeping bags (to my knowledge). It's not "waterproof" but is highly water resistant. I think it's a great compromise for a truly cold weather bag. Gore replaced my delaminating WM "Dakota" Goretex model with a WM Antelope with a GWS (windstopper) shell. It was a great bag, and I used it until I got a WM Versalite. The GWS, while more breathable than normal Goretex, was still not breathable enough for my liking and limited the bags temp range. GWS also adds a fair bit of weight to a bag.
Only a few bags have actual WPB shells anymore, none of which are Goretex. One is Pertex "Shield", another is a Mountain Hardwear's proprietary fabric called "Dry Q Elite."
So that's the long way of saying calling a Pertex Shield bag or Dry Q Elite bag "Goretex" is a misnomer...
