I've actually driven two E53s in Canada and they were identical to the U.S. vehicles in every (noticeable) way. I can't think of much else such a flash would change aside from the OBC. The only other logical guess I could make is that it would alter the emissions systems in some way (i.e. warm-up sequence duration, fuel delivery at idle, secondary air-pump activation and duration, etc.).

One final thing comes to mind: the fuel grade in Canada only goes up to 91. Maybe the flash changes something to do with ignition timing to better adapt the car to 91 Octane. Who knows.

If neither of those are it, your guess is as good as mine - and I'm Canadian!