I've been using the Neoair Xlite Large, trimmed to ~60in, for my last couple of outings. I have found that it is slightly less comfortable than the Synmat UL7 LW I previously had. However, I have had 3 Synmats leak on me now, 2 of them on their maiden voyages. I'm familiar with getting the valves seated properly, but too many slow leaks converted me to a Neoair.
Here's what I've found regarding comfort: the Neoair is MUCH more comfortable if I slightly prop it up, from underneath, at the head and near the knees/lower legs (like with a sit pad or piece of ccf, clothing, some leaves/duff in a trash bag, whatever). For some reason, this makes the pad much more comfortable for sleeping on back or side. I think it allows the horizontally-baffled pad to contour the body a little better. Like the OP, I found the Neoair on a flat surface rather board-like. But with the above tip, I sleep just as well as the few nights my UL7 didn't flatten. Since you already have an Xtherm, it'd be pretty easy to give it a try.
Also, FWIW, a large Xlite trimmed to 60in weighs 13oz, and I can fit my whole body on it when sleeping on my side. And it was very easy to trim.