I've been mostly absent from BPL for the past couple years; coming back, I've definitely noticed the (as you put it) "mixed feeling on this forum about the sense or logic in trying to reach a certain base weight goal." I don't remember these "mixed feelings" being so prominent before. On the other hand, I have seen recent posts where desire to hit a certain weight seems to occur in a vacuum (as it were) and the poster seems to have little actual backpacking experience. I don't recall posts like that in the past, either.
Setting a nice round number as a target weight for given conditions has always made sense to me--it's a tool that encourages careful scrutiny of every item and (ideally) leads to cutting weight by leaving out unnecessary, redundant, or "pack your fears" pieces. I still think that's a key part of "Pack Less. Be More."
I use a clothing system almost identical to yours for similar conditions; I don't consider it an SUL kit, just a light kit that works well with nothing extra and nothing left out. I've done an SUL trip in the past, but without going to cuben for tarp and pack, I have to carry a CCF pad to hit the magic 5# goal. One trip was enough to convince me I can't sleep comfortably on CCF anymore. Sure, I could spend the money to replace perfectly good gear with cuben, and be able to spend the saved weight on a decent pad (which I already own), but I doubt I'd feel the 8-10 oz difference on my back.
Regardless, I still keep an SUL list--I still think it's a very valuable tool for all the reasons above. With regard to your specific questions:
> For the vest to save you any real weight, it seems like it will be essentially the same loft and fill weight around the torso as the Montbell, just without arms. So you give up actual warmth and insulation in exchange for a lighter piece. The question is, what were the down sleeves doing for you? It seems like you could experiment by using a cheap fleece vest (of equivalent warmth), either around town or on a short trip.
>Jacket under vest or vest under jacket seems like a matter of convenience more than anything else. If you anticipate hiking in your rain jacket, obviously it's easier to keep the vest on top.
>My understanding is Propore jackets breathe about as well as eVent. My FroggToggs jacket weighs 5.4oz. Of course, the fit and durability are poor. But I just picked up a Montbell windshirt (2.3 oz) which fits well and is more breathable than propore. Combined, the two pieces weigh less than your eVent jacket and are (I suspect) more versatile than the ZPacks jacket alone. Like the poster above, I rarely wear rain pants, but my Frogg Toggs pants are actually a hair lighter than the zPacks (3.7 oz) and would just fine for occasional use. Maybe wind shirt + propore + ZPacks rain kilt?
>Patagonia's long sleeve 1/4 zip merino/poly top (what you already have, I think) is a fantastic base layer in my experience. Warm enough for cold temps, cool enough with the zip down and sleeves pushed up to handle hard hiking into the 70s without overheating.
>I've been thinking about sewing up some rain mitts out of propore--either cut from an old jacket or from one of the reusable grocery bags that seem to made of it. Should be as light as cuben, and cheap enough durability won't matter much. The rain shells that came with my OR Versa gloves only weigh 0.7 oz (not seam-sealed) so that's another option. Like the other poster, I rarely wear shell gloves or mitts, but would want gloves that retained significant warmth when wet if I expected cold sustained rain.
You obviously know what you're doing and I don't know how helpful any of what I have to say will be. I did find your post very helpful to me, to think about other options for my own gear in similar conditions. Thanks especially for your second post, detailing the conditions where you needed rain pants and mitts. Gives me something to think about.