I dragged my RN submariner jumper (made by Outdoor Knitwear) out today as the temperature is really starting to drop here and TBH I think as a design it's the perfect cold weather jumper. I've got lots of other ones but I always come back to this, for me it's perfect under an A-2 (so long as you have room for it - none of that super trim jacket nonsense), Aero Barnstormer, Irvin, Wehrmacht winter parka, waxed cotton coat, etc, etc. The roll neck and longish body keeps the wind out.
Yes there's lots of other things you can wear under an A-2 or other flying jacket but I'd bet there's none which are so effective as a RN submariner jumper or other decent thick roll neck jumper.