Like everyone else is stating, it would flatten it out to where it's worthless. I own 3 UQ's and love them all for different scenarios. Arrowhead synthetic for damp trips, HG 20 degree Phoenix for 3 season and just picked up a Loco Libre Ridge Reaper zero with overstuff for winter. They go on so easy with very little fiddle factor.
I tried using the foam pad from Gosseamer Gear in between my RR but it was more of a pain than anything. You feel every crease and wrinkle.