I think you'll like it. I run an down vest with a teton puffy. For the same reason. The vest can fit under or over the jacket. Neither way has a performance benifit, just depends which one I start with.
Down can be a sweat factory but with a vest heat dumps out the arm holes. I assume the uncompahgre vest would breathe better because of the nature of the 37.5 treatment.
You will have better wind protection with the cirrus than I do. Also you won't need a rain jacket as often.
Over I like having an insulated vest instead of a second merino, poly or fleece layer. More warmth for the weight and a puffy vest isn't so loud when worn over a shirt since it doesn't have noisy sleeves rubbing on the body.
I personally like the quietness of the teton for archery season, otherwise I'd look into a more traditional puffy like the cirrus.
For the OP I've also worn my down vest under a midweight fleece jacket. Pretty toasty, quiet, and durable. Down side is my particular fleece is heavy at 23 ounces, holds burs, and the typical melting near a campfire.
Does anyone know how the new sitka kelvin active may fit the needs?