Interesting - i found myself with an instant reaction on reading the Question, then see-sawing my opinion as I read through the replies.
I think the idea of trying to make actors look 'authentic' can only be carried so far - as pointed out, with Hollywood-level prosthetics and talent, Eddie Murphy can look like an old jewish gentleman, but that's hardly likely to be expected, much less delivered, at this level.
Simplest would have been to simply allow costume to indicate ethnicity, saves issues all round in the circumstances, and let's be clear, it is the circumstances that dictate what would be appropriate.
I have never seen SP performed on stage, but I found the film alternately ludircrous, sinister, boring, tasteless, strange, and with a gigantic homo-erotic undertone - try bringing that to realisation with an anateur cast you'd find the issue of skin colour way down the list of problems!!!