Yeah, Luke's performance against Vader impresses me a lot more than anything Kas'im has ever done. Sure, he mastered a bunch of forms - so did Bondara and Shaak Ti. Luke actually displays a more prodigious saber feat in matching Vader in a duel with like a month of training, and then later copying his moves and matching him evenly just a few months later. Kas'im, meanwhile, was being driven back by trainee Bane's sheer strength in the dark side - a strength that Vader would laugh at.