I suggest this be a setting in the filetype file, not a hard coded magic list.
Agree. And the user must be able to override this.
For Latex, I definitely disagree with this change. When I used Latex for my master's thesis I enjoyed auto-completion for my custom macros as well as long words.
Generally I'm not sure this change is a good idea. If you don't need auto-completion then the pop-up is relatively easy to ignore or cancel if it obscures something important. On the other hand, if you enjoy auto-completion it'll be very hard to get it back.