Let's say income per-capita is expected to grow 50% faster in China than in the US over the next thirty years?

Where do u come out with the 50% faster growth figure from? I don't think that is a very accurate figure at all, and any answer here is going to be speculative. However if this did happen :s then i would expect the exchange rate to increase in favour of the yuan by an amount (dependant on the relative value of the dollar).