Wlad's chin seems to have improved, though he rarely gets caught with anything big since Peter and with Manny as his trainer he really has improved his defense and has perfected his jab and grab style.
I think Cotto took better punches at WW because his body was more naturally suited for that weight and he wasn't weight drained.
i will, Nigel Benn's punch resistance seemed to improve as he aged, if my memory serves me correctly Benn was considered chinny early in his career but it seemed to get better - maybe some of his english fans can shed some light on this