Keep in mind that the best leg-spinner of the world was largely useless in India against India. Since Warne is >> McGain, I'd expect maybe 2-3 wickets maximum from McGain. However, he'd probably have kept the runs down a lot more.
Good point you make. Still I wonder what he could have done in the 2nd test after the success Mishra had. He certainly would have been a lot more threatening. Would have been a handful on this wicket as well.