When I've used flat full underquilts, the fit issues I've found are along the legs...the UQ is snugged against my torso but there's a small gap under my legs. Cold legs mean cold feet.
I've found that partial UQs have fewer fit issues...which means I have warmer feet.
The full UQs that are darted or differentially cut have fewer fit issues than the flat ones...mainly b/c you can snug them up as tight as you want to and the down isn't compressed.
So when I've had a great fitting full-length UQ, and when I've used partial UQs with pads under my feet, I've been very warm. It's more about the fit of the system than it is about a pad or an UQ individually.