I just got around to seeing this. I think Viggo and Ali elevated what could have been a very corny movie. The performances by both actors was excellent and they had terrific chemistry. Look I'm sure there were a few liberties taken with the story as is the case of most biopics/historical movies, so this is no different. Marershala Ali is quickly establishing himself as one of the best actors out now.
There are very few actors that could've elevated a movie this average into what it was.
As for the movie, I agree with you. I doubt there were more liberties than any other bio pic. But I don't think the movie insulted, or portrayed Shirley as anything less than a proud black man. From all account, he certainly had his ways about him. Hell, I think it may have been closer to Lip's version than whatever his family thinks.