I have put a sleeping bag or quilt down in the UC, under a space blanket, and it has been very warm. The only problem is a tendency for the elastic UC to compress the insulation in some spots.
I have used the WM blue CCf pads and the Thermarest Ridgerest, but they don't curve around your body or work nearly as well in the HHSS UC as does the HH ocf pad. They work better sleeping right on top of them, and then they are pretty warm. But I don't think they are any warmer than the HH UC and pad/ space blanket, not as comfy and take up a whole lot more volume than the HH system.
Have you tried a compression dry or stuff sack with your HH pad? Once just for fun I got my HH pad and the UC back into the sack they came in:
Now getting it back in there would be way too much hassle for me on the trail, I just squeeze it all down into a dry sack or my pack and compress it way down that way. And I don't try to roll it up tight or compress it with my hands, too easy to polk a hole in it. I put it loosely in a sack 1st, with or without my down clothing, and then compress the sack. Just like I do with a Pea Pod or UQ. ABut it does show how much it can be shrunk down. Way more than any CCF pad I have. In fact, I'm no sure an equally warm and wind proof ( i.e. with an UQ protector ) down UQ would pack down a whole lot smaller.