The reason why British Airways and Japan Airlines favor the 747-8 over the A380-800 is the fact both airlines control a large fraction of the landing slots at their respective home airports (London Heathrow and Tokyo Narita), so there is no pressure to buy as big a plane as possible.
Somewhere I read the range will be over 8,200 nautical miles. Considering Boeing's reputation for underestimating the actual performance of its planes before they are tested, I wouldn't be surprised if it were actually a bit more. Even if Airbus comes out with a high gross weight version of the A380-800, I doubt it will match the CASM of the 747-8I.