I’ve never tested the theory (I tend to avoid going out in really cold conditions)...but guess something like a Paramo Torres Gilet where you put it on over your waterproof layer...rather than a fleece where you have to take off waterproof layer..maybe getting drenched and cold while doing that.....might be a better solution for walks where you need some extra warm for some sections.
I can confirm that this is a very good option.
1. It helps protect lower layers from being exposed to the rain on changing.
2. It encourages earlier addition of the extra layer as it is so quick and easy.
3. If your main outer layer is also Paramo it will suck away some of the water in it. Many's the time I've taken my Torres off after a cold hard shower to find my Velez totally dried out.
Which reminds me, the conditions described are where Paramo excels. Goretex keeps you dry up to a point but then you stay cold and wet. Paramo let's the water in earlier, but kicks it back out again quickly.
Then, of course, there's Buffalo which stays warm whatever.