How “waterproof” is Dynema X?

It's as waterproof as most PU-backed pack fabrics. It will repel rain very well until the factory DWR wears off due to dirt, which happens pretty fast, and then shed rain decently until the coating degrades from internal abrasion and/or UV, at which point it will leak easily. Golite used some partially recycled gridstop in 2011-2013, which has a distinctly sub-par PU coating. The standard gridstop (e.g. the Thruhiker stuff) has a PU coating as good as anything short of cuben or dimension polyant fabrics.

My experience with PU fabrics is that under serious stress (lots of heavy rain or wet snow, packrafting through rapids) they do leak directly through the fabric, in a diffuse manner, and even when new.

What David said. I assume no pack is waterproof and needs a pack liner if in really rainy conditions and/or a rain cover. I use a pack liner most of the time here in the east.

Thanks Alex and David.

Cool. I'm guessing that with a poncho or gatewood cape then, I shouldn't have any issues with my Zpacks ArcHaul when it arrives.