Since the current 2 Heathrow Connect trains per hour continue to T5 empty anyway there is clearly the capacity to run the 4 Crossrail trains to T5.

I do wonder how many people will actually stump up for Hex from Paddington when they can jump on Crossrail in the West End, City or Docklands. It really will just be of use for areas with very quick connections from Paddington.