I like these field reports; hope to see more in the future. The notes/reflections on what worked well in given conditions is valuable information for others to help them further refine (or re-think) their own personal kits.
Like Jhaura posted above, we've been doing some trips here in our local backcountry this winter, trying to time them with winter storms so we can get some snow and/or rain. Our wintry trips match pretty well the conditions you describe in this report: temps 20s-40s, rain, little bit of snow but generally not enough for snowshoes, deadman anchors, etc. We're finding our 3-season kits with some slight modifications work quite well for us.
I'd be interested in learning more of the details of your synthetic overquilt and how you pair it with your down quilt. My 15*F Katabatic keeps me warm to the mid teens with just base layers on and I know I can push it farther with my insulation pieces on, but I've been thinking about having a synthetic quilt made that can do double duty as an overquilt in the winter (ideally I'd like to push my sleep system comfortably down to ~0*F) and as a stand-alone summer quilt for our late spring through fall. I've noticed a lot of moisture accumulation in my down quilts on these winter trips so the added moisture managing benefits of the synthetic quilt is intriguing to me.
A couple of questions if you don't mind...
Any sense of the approximate warmth rating of your synthetic quilt by itself?
What sort of dimensions did you need for it to work well as an overquilt? I'd assume a little wider at minimum...
How did you rig it to stay attached to, and on top of, your down quilt?
I'm looking at the EE Prodigy Quilts in a Reg length/Wide cut at the 50* rating as my possible solution, so I'm just curious how this compares to what you (or others) are using.