Looking at the stats for undead and the Ethereal ability says to add the creatures Willpower to its soak vs non-magical/divine attacks.

The Cairn Wraith shows a soak of 3 and a Willpower of 3.

The Spirit shows a soak of 4 and a Willpower of 4.

So.... has the soak value already been increased by the creature's Willpower for these creatures, or will non-magical/divine attacks be going up against soak values of 6 (for Cairn Wraiths) and 8 (for Spirits)?

How impervious are these critters, anyway?

I would say it hasn't already been factored in as you would expect it to be hard to hit something ethereal with a regular weapon based on previous editions and other systems so 6 sounds about right to me

I agree with pumpkin. The additional soak has not already been factored in. So, vs magical weapons their soak is 3/4, vs non-magical their soak is 6/8, etc.