Most likely the difference in ISO is a result of the different chemistry in the development. ECN-2 may be less active than C-41. Rem-jet shouldn't affect exposure or development, except to the extent that it suppresses halation in the camera. Their "premoval" process just saves the user the hassle of scrubbing the remjet off. And saves conventional labs the hassle of having their machinery gunked up with gloopy black stuff. (Note the phrase "safe for processing in standard photo lab machines".)
I've encountered stories of people loading cinema film in still cameras and then taking it to a movie film lab for processing, but I expect that's an expensive way to go: I can't imagine that the lab would welcome having to splice my 3 feet of film onto their 10,000 foot run. They'd have to charge a lot more than a conventional still film lab to pay for that hassle.
But the coolest thing about 5219/7219 is the 16 stops of effective latitude: at those numbers, ISOs become pretty arbitrary. A little push or pull in the soup and you've got a huge range to play with.
