As we all know, HR was in negotiations for this movie for a long time before Aamir signed on all of a sudden.
In my opinion, the character himself was fine, but the constant side-changing was irritating, and Aamir did not suit him at all.
Having seen the movie, do you think HR would have been able to do this role better? I personally think so. I consider him a more versatile actor than Aamir, and in action scenes he would have been able to be more convincing. As far as comedy is concerned, I don't think we have seen HR in an outright comic role ever, but the kind of comedy Firangi is supposed to do, I think he could have handled it.
More importantly, with HR, Fatima wouldn't have been cast in that important role. It might have been played by an actress who could do it better, and that would have made the movie better.
Aamir himself said that Fatima had the main role, it is possible that the feedback to her performance would have lead to her role getting cut.
HR would have done it better