I'll defer to Quoddy's first-hand experience with the Yeti reflector add-on, but be advised that the Yeti 2/3 length UQ comes in two weights, 3 season and winter. The 3 season Yeti without the reflective add-on is good to around freezing or bit below. The Winter weight should be good into the low teens or a bit lower. The add-on liner would supplement those to low-20s and zero, respectively.
With the Yeti, you'd still need to add insulation under your legs and feet, though many partial pad solutions will work. You'll still have to deal with coming up with enough R-value to handle the temperatures you'll be facing...er...sitting...well, you know!
FWIW, the MW4 down UQ from JRB has 3.5"-4" of loft and is rated "0-10 degrees" Fahrenheit. It is a full-length UQ so no additional feet/legs insulation is needed.