To be honest I dont think Pac can 'prove' he's better than Mayweather and vice versa, unless they fight each other. Otherwise it's all subjective. One can probably end up saying that he's 'greater' due to records and acheivements and what not, but that doesn't neccessarily make a better, more skilled boxer.

As for knocking out Mosley, he's knocked out or TKO'd other fighters after Floyd fought them... they have different styles, end of.