I haven't used it as an underquilt but I've strung it up to show my son that it could be done.
I bought a 20* Rev X with a 20% overfill last summer, in my experience the quilt is very well made and has performed well - used it along with a fleece liner to 3* at the NJ Winter hang and was very warm.
I too have the 20° RevX with 20% overfill. I'm very satisfied with it's build quality and with its performance. Coldest I've slept using it was 14° (I had on some extra clothing layers as well) and got a good nights sleep. It works for me, however I'm considering getting another topquilt for warmer weather. The 20° rated RevX can be a bit too warm in balmier weather.
I have a few quilts from EE and have been really pleased with their performance and the workmanship. The only complaints I ever heard had to do with the warmth rating in the Revelation series due to the spacing of the baffles. If the quilt was only stuffed for say 40*, then the down would shift off the highest point, like the user's hip or chest. Higher fill volume forced more of the down to stay put so the warmer versions weren't affected. The baffle spaces are smaller on the new quilts to address that issue.
The Revelation is very similar in build to the Prodigy, it's just a matter of fill. The Revelation series is down with Karo step baffles. The Prodigy is done with Climashield. If you have any trouble with the quilt you choose, Tim will do right by you and make you one that will suit your needs. All of my quilts have shipped on the day I was quoted when I purchased them. Nothing bad to say about EE.
I haven't been lucky enough to own one of these quilts yet, but from what I can see on BPL Tim Marshall is about the best there is when it comes to taking care of his customers