At least in the current edition of Harnmaster normal blow has 26% of hitting chest or abdomen but only 9% chance of hitting hip or groin. The URW system is a bit different, but I doubt it's too different. In addition abdomen is a weak spot more vulnerable to mortal wounds than most body parts (which, based on personal experience, seems true in URW as well). Plus hip and groin are likely to have good protection anyway since both torso and leg pieces tend to double down on it (cuirass being a rare exception).
For warmth I do know that your hands and feet are the first thing to freeze given equal warmth coverage. Since that's realistic (blood being diverted from extremities to vital functions) I'd expect the rest of the system to not be too far out of line. I suppose testing how far you can get while neglecting to stack clothing to areas like knees, elbows and shoulders would be interesting. E.g. woollen dress, linen dress, nettle cloak and furs for the extremities, I do suspect it will have worse result than switching one of the dresses for an undershirt would as the game does seem to take some issue with entirely unclothed body parts.