Goosefeet gear down booties and sleeping bag temp rating – how to extend comfort range of bag?

it depends- are your feet getting cold? if they are and everything else is fine, the GF booties do a very good job of keeping your feet warm, if you're cold overall then I don't think the booties are going to do it

same goes for down pants, they do a great job keeping your lower torso warm, but if your upper torso is cold you're going to have to address that as well

if you see yourself finding down pants and booties (get the overboots too for outside use) useful for lounging around camp, it makes sense to use them to stretch the range of your bag; if your sole purpose is to stretch your sleeping bag you're better off investing in a warmer bag imo

I find that my hands/feet are very important to comfort. My perception of being "cold" or warm is highly impacted by my extremities. The booties make a big difference in what I consider comfortable so they are well worth the 1.8 ounce carry weight.

In terms of giving you an exact number there are just too many variables. The outside wind/humidity has an impact not to mention your health, hydration, and your stress/comfort level all impact how you feel. But for me…generally they add several degrees of comfort to my sleeping system.

You didn't mention your sleeping pad. Depending on what you use now you might be able to get a few more degrees out of your Zpacks bag by upgrading there or adding a thin CCF in cold weather.

I find my Goosefeet down socks (with 100% overfill) almost too hot. If the temperature drops low enough for me to put the socks on, the rest of my body quickly becomes too cold without a large down parka and insulated pants. I have found that it's nice to put the socks on when I first go to sleep if my feet have been wet/cold during the day then take them off once my feet warm up.