I still use a mummy bag in the hammock and haven't yet had a need for an underquilt. Last night I took my new montbell bag down to 31 degrees (its rated to 30) and was warm and comfortable. Underneath I used a full length thermarest zlite and an insulated jacket under my butt.
I can't talk myself into fully committing to the quilts because I am still attached to the bags and I like to go to the ground every now and then when I am with company that doesn't do the hammock thing.
I got a older ripstop nylon down sleeping back im turning into a under quilt just to try it with my mummy bag