In reply to Cameron94: Maybe I am bit heavier than you with my sac on but the PU insert compresses as the heel hits the ground or snow and the black rubber sole takes on a rounded shape.
This is from the MCofS site below by Roger Wild, the point I am making is these PU inserts do not make a relatively stiff soled and safe boot in decsent.
MCofS
Boots:
Boots need to be relatively stiffsoled with sufficient rigidity to create secure footsteps by using the side edges or toes. They don’t need to be heavy or fully stiffened mountaineering boots but if you can bend the sole easily in your hands then it’s too soft and bendy to create secure footsteps. Rounded heels are no good because you can’t create a firm step when descending. Don’t be fooled into thinking that stiff soled boots are only needed on steep slopes. Many slips and falls occur on relatively easy angled slopes on popular hillwalking routes.