when winky wright was young he had a very bad chin, the guy had no punch resistance at all. he sorted it out by becoming a very good defensive technician. once his confidence was orite then his chin slowly began to improve. i saw him getting dropped 5 times in a fight, and then when he fought shane mosley he dropped his hands and got hit but it didn't shake him.
Wright has only ever been on the floor against Vasquez.... three were slips. He wasn't prolifically weak chinned.
JC Vasquez had remarkable punch quality. Going down to him but getting back up is no indicator of a weak chin. The guy had one punch KO power which boxrec may or may not suggest.
The historical perspective: It is certainly possible that Khan's chin will improve considerably based on what has happened to previous fighters. It is also possible that it wont. We will just have to wait and see.