Having sat the Mensa tests (they were in the neighbourhood, curiosity got the better of me), I don't think they're an indicator of much other than your ability to answer pointless logic problems quickly.
Personally I think work ethic is always going to be a bigger factor in peoples success than IQ, although obviously a combination of both would be ideal. I was admitted to Mensa, but have average academics (real world average, not "I only got 10A* grades" TSR average...), so in my experience there isn't much correlation.
That said, there may be better professional tests out there.