Cold Weather Sleep Setup - Single Very Cold Bag or Quilt/Bag Combo

I'm preparing for an cold weather car-based tent-camping trip this winter which might experience -25F and snow.

I currently use an REI Magma +15, and have snow camped in it before (a little uncomfortably) and picked up a Mammut Tywin -15F bag on sale. The Mammut is tight and very heavy, but is incredibly well-made and a nice DWR shell and a single-component system.

To reduce weight and increase comfort, I've been thinking of ditching the Mammut and instead going with my Magma, a liner, and purchasing EE Revelations 20-degree over-quilt. I also have a cold-weather pad.

Do you have experience using an over-quilt with a sleeping bag? What do you like with multiple components vs. a single component very cold-weather bag?

I've never used an overquilt.