I never batted an eye lid at Bennedict playing Khan. Beyond the name his ethnicity and cultural background had nothing to do with the Khan character really. BC was just a great actor to be a villain. As has been said above Montalban wasn't Sikh either. It's not like BC pulled an Alec Guinness and blacked up and put on a voice...

The original TV Khan is arguably more problematic as Montalban applied a thick layer of skin darking makeup...

...something they wisely dropped for the film Khan. But in this day and age there's no excuse for having Depp in The Lone Ranger doing a "We in heap big trouble kemosabe" voice. And no ammount of photo oportunities with Native American delegations is gonna change that...