The pack I now use the most is my Zpacks custom Zero in hybrid nylon/cuben fabric, which itself is waterproof, but I wonder if the nylon on the outside soaks up any water... doesn't seem to from the 7 or 8 times I have taken it out, half of which was in some kind of rain (one time was out in heavy rain for about 12 straight). I seam sealed it from the inside to avoid having to use a pack liner/garbage bag, plus the stuff sacks I use are either sil-nylon or cuben, and very water resistant. So it also depends on what kind of pack you use, too. Some won't soak up very much water, if any at all, with proper seam sealing and/or waterproofing. I should also note that I only had to use a small fraction of the tube of goo, maybe one fifth or sixth, there were not that many seams on the inside of my pack.
I do own a Zpacks cuben pack cover too, but I use it for my urban pack, as I often will haul around my lap top and it rains a lot where I live.
If I were to go on a longer trip, like a thru-hike, I might opt for my MLD Exodus, in which case I would take a poncho to cover both me and the pack, plus I would put all my night clothing/extra layers along with my down sleeping bag inside a 20l STS dry bag. I've tried the whole garbage bag liner thing, and didn't like fussing around with it--I like just being able to get to everything in my pack without having something else in the way, and one less thing to worry about while packing, just throw it all directly in the pack. What is important is just to keep the sleeping bag and clothing dry, I don't really care if my Ti pot or my tarp/tent get wet, and my food is mostly in ziplock bags anyhow so it will stay dry.
Anyhow, if you are looking to save the most weight vs. wet pack, get a cuben fiber pack and seam seal it.