I have the SKRE ptarmigan without the hood that I wore for deer season in December. It was like 13 degrees. I had a Sitka mid layer and T-shirt on while walking but when standing I would slip the ptarmigan on and was plenty warm and comfortable. I don’t think I’d wear anything over top of it unless it was raining or something.
I think fit can dictate this, but I always try to keep my down puffy layer on the outside - regardless of # of layers underneath. Only exception is rain shell if raining or really windy.
When I buy bottom or mid or outer layers I always keep in mind the way I want to layer them in the most different weather situations.
Goose down and other lofty fills are most effective when they can loft to their maximum. Wearing them as the last layer allows that to happen. Wearing other layers over them can restrict the loft or actually compress the puffy and reduce the insulative value. I always wear mine on the outside. Doing so also allows me to add/remove it as needed with the least hassle.
The only thing I would wear over a good down jacket would be a waterproof/windproof hardshell that fit loose over the jacket. Something like that would add warmth for sure.
As above, with the exception of wet weather and noise, I think outside is best. Down garments and puffies loose a lot of loft when used as a mid layer. Reduced loft greatly reduces warmth. I really notice the difference on my FL uncomphagre when I have worn it under a shell or under my Woodbury coat. It squishes down and provides very little warmth.