Personally I find the 2.5″ thickness of the last generation X-Lites to be optimum, yet there are lots of backpackers who I’m sure want the extra half inch for a more plush bed. One of the reasons Exped has remained relevant (also flat valves) is because they’ve maintained 3″ plus thick pads in their lineup. Whether it was a good move on Thermarest’s part in terms of sales to go with more thickness on the X-Lite and X-Therm NXT’s only time will tell. However it’s the big reduction in noise which could be the main selling point of these new pads.
With a true elongated A frame shelter like the TT Protrail, Yama Cirriform or tarps I don’t think the 3″ pad will matter much in regards to head and foot room. But with mids that already make it seem like the fly is in your face anyway, the thicker pads could be an issue for sure, especially if you’re taller.
Matthew, as far as rectangular vs mummy shape on cutting down an X-Lite I don’t know. I’ve only modified a Klymit pad, but for a better idea of what to do I’d PM Eric Blanche who posted earlier in this thread because he’s actually done a successful X-Lite mod and the guy is very savvy at DIY.
Not sure if you specifically had in mind the new NXT version Matthew, but if the last generation X-lites are adequate then right now campsaver has the regular/wides for 30% off. https://www.campsaver.com/therm-a-rest-neoair-xlite-sleeping-pad.html?_iv_code=2XV-SLP-THR0226-13214