This is my second season with a medium, at 6'1" and 185lbs, usually a 32/34 waist, I think I'm on the edge of needing the large. I don't have any issue with layering, SE Michigan can get pretty chilly.
A buddy got a large, and I tried it on... not like I was swimming in it, but it definitely felt roomier. I never sat in his Mantis tho, just tried it on.
I have zero complaints about my medium. Any comfort issues like hip pinch, riding up, foot fatigue have been easily mitigated by tether height, length and or bridge adjustment. I did swap the fixed length bridge for a diy utilibridge to accommodate bridge adjustment.