Performance implications of many user roles
I don't have any hard stats but based on several years of experience and knowing how Drupal works I would be extremely surprised if adding another user role made any appreciable difference at the page request level. The only difference will be when a user has multiple roles, but the amount of extra data required per role is tiny.

I have profiled page loads for all sorts of different site setups and never seen an issue relating to the number of user roles.

Having said that, adding extra roles can make administration more complicated, so you won't want to add new ones without a good reason. If you were to create a hundred user roles the main permissions panel would be unusable but I would be willing to bet that the effect on normal page loads would be negligible.