I have no personal experiuence with the modern materials perform: I have seen some failures (e.g. a G-T bivy at -25 C was an epic fail), but that does not mean that the stuff isn't good when used appropriatly. As to skis I'm afraid I only have experience with two types of skis; Tegsnäs and Vita Blixten ("White Lightning", the nickname for the old wooden Swedish army ski). It is very possible that the modern plastics actually can deal well with +2 C; I was never able to get Tegnsäs to do so (but that might be my failure; is there a magic non-stick wax for slushy snow?)
My personal kit appears to go backwards in time rather than forwards; by the time I die I'll most likely will make Ötzi look like a hi-tech gear-head...
I do agee that you need to know what you are doing at -20 C (which I pointed out). But once you do know that (and have the proper kit) then -25 C is easier than -5 C. IMNSHO.
Well, that's not much of ski experience. You should try modern steel edged back country skiis. My Åsnes Rago are light years better in every single way than the army skis. I have only tried Tegsnäs once, and they were okay in loose snow, terrible on frozen snow. Have you done vasaloppet? I have twice, once in the army on white lightnings, and once with skating skis. Guess which pair that got me the best time?
I wet/cold conditions, I prefer modern materials any day. There is no traditional stuff that can perform in the vicinity of ptfe membranes and synthetic insulation.