T5C seems to have already opened. I arrived from on the BA flight from GRU on Tuesday morning and we were told we were the first aircraft to be using T5C.
It was pretty difficult to navigate, no one really seemed to know where they were going.
Once you find the transit train, it whisks you to T5A to clear immigration and collect your baggage, stopping at T5B en-route. It was all pretty painless to be fair.