'I'm outta here' is very informal. 'I'm off' isn't quite as 'committed' - 'I'm off in five minutes, I've just got to ...[last activity] ... Right. All done. I'm outta here. (If the phone rings after you've said 'I'm outta here', you'll probably ignore it!)
I've never heard 'I'm out' in this sense (though it's used with several other meanings - 'I've exhausted my supply/run out of something'. 'I won't participate in that venture', 'tell callers I'm not at home'...). Perhaps 'I'm out' does mean 'I'm off' in some parts of the world though.
b
