Many of the better ($$) pads will claim a "R-value" for insulation rating. But as you say, if it gets compressed enough, that r value is meaningless. I usually carry a thin blanket in addition to the sleeping bag for added "r-value", or, I use it as a pillow.
Speaking of pillow, after having some neck issues and being diagnosed with mild stenosis, I bought one of these: https://www.amazon.com/gp/product/B01LW4CRSM/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1
It helps a lot and folds away to very tiny. Only downside is it's kinda noisy whenever you move.