Tom, thanks for sharing your experience. if there is stronger clamping force with the 100s, i might have trouble with them aswell, even with XL pads. P7 aren't even strong in clamping and get my head aching. the only on ears I had are the sennheiser px200 which I really liked at their time but they weren't really durable. they didn't ache at all, pressure was nearly absent and still they stood on the head without a problem. thanks for reminding me, I almost forgot them. if the experience is comparable they could be perfect.
However, from reading others reviews on the p7 it seems that they dont use memory foam for the earpads, plus the leather they use there isnt very pliable. Even looking at the reviews on youtube with the m100s with the xl pads u can see the reviewer squishing the pads and they really squish well, does look like very comfortable. The m100s headband is known to take a beating as well so if anything flex em out. i only tried the p7 as a demo at futureshop which is probably not accurate but it wasnt the most comfortable over ear. Pads were pretty stiff.