I would say running EMUs for almost 500 kms will be bad idea not least because it does not have toilets on board plus after paying that sort of fare, no one would want to stand through the journey so that puts the capacity down to just around 80 odd per coach. So that makes it a 12 coach AC Express, just like the Double Decker but with reduced capacity. Although if given a proper timetable and speed it would not be too far fetched to start a regular EMU service between Mumbai and Vapi instead of a passenger. It would definitely save some time as EMUs have a lot better braking and pickup, plus with the new Bombardier EMUs now using regenerative braking it would bring down the electricity usage too.