Best practice for importing thousands of users?

I see that we can select how many to process each time a file is imported. I am assuming that the module will not import someone who is already in the database, so it will basically do 250 (or whatever I set that to) at a time.

In my case, we have a number of roles that will be imported - and I will only send emails when importing the list of authenticated users. Now will an email not be sent if a user is already in the db and just having the role added? And will each new import get the custom email, or is this still an issue (as seen here http://drupal.org/node/468928)?