You could call WM and ask them about the warmth of an overfilled bag.

Three things (at least 3) happen on overfilling a down bag. The loft increases a little which adds warmth just from thickness. The down compresses a little which means the density goes up a little and that means that the thermal conductivity decreases a little which adds warmth. The weight of the bag goes up by the amount of overfill, and the warmth to weight ratio can be changed by an unknown amount. Oh, the cost goes up too.

Your best bet is to call WM and ask. Or, hear someone talk about personal experience. Or buy a bag made to 30F or 25F or 20F specifications.