I sure have. I still have a lot of experimentation to do on that front. But I have put the IX on top of the pad and got a major boost. Since my IX quilt was designed specifically for an HH Explorer, the fit is perfect. I have experimented a bit with replacing the pad with just the IX, but don't yet have conclusive results. I'm thinking the pad/space blanket was a bit warmer than just the 2 layer IX, both inside the UC. But I'm not sure yet.
I also placed this same IX UQ inside a PeaPod on a 26*F night. With no TQ, but wearing warm fleece clothing and using a light puffy jacket to seal around the neck/chest as needed. I was mostly way too hot that night, and had to do some major venting. Previous experience with a similar set up but no IX, and I was just barely warm enough at about the same temps, while doing almost no venting. So I guess it was the IX under me that caused a major boost in the PeaPod.