I am about to install underlayment under a standing seam aluminum roof. The climate is borderline zone 4/5 but near ocean with a lot o wind driven rain but not much snow. The roof is an unvented cathedral with 10" of cellulose in the rafter bays, 5/8" zip sheathing, two layers of 2" XPS with joints staggered and 5/8" CDX screwed through the foam into the rafters. The roofer recommends Grace Ultra peel and stick over the whole roof but says Ice and Water at the valleys and eaves with a permeable underlayment is also acceptable. Due to our wind driven rain I like the idea of using the Grace over the whole roof but I'm a bit concerned about eliminating all the drying potential to the outside. The ceiling finish will be gyp board painted with latex paint (no vapor barrier). Any thoughts on permeable vs non permeable underlayments? By the way, is there any big difference between the permeability of gyp taped and painted vs blueboard and skimcoat plaster?

Vapor barrier on the exterior wouldn't be my choice in a cold climate. I'd keep it vapor permeable (however minimal) and use furring strips to create a vent.