Windshells will not keep you dry when it rains. They might repel a short drizzle, but not real rain. Remember that with the winds, the rain will be horizontal, so ponchos or omitting rain pants are probably not a great idea.

Breathable rain gear can substitute for windshells, but not the other way around.

Added later: In those conditions I'd want a lightweight (100 wt.) fleece top as a mid-layer for hiking. You don't want to be wearing down when actively moving. Your perspiration under your rain jacked will definitely compromise your down insulation. The down is for when you're resting, in camp and in case it gets too cold for your sleeping bag at night.