in general it should perform fine with that many pages and roles.
page management and role management might be a but unwieldy, we have an add on in our store, Page Manager Pro that can help with the page management.
the site map will be cached in memory and it is a little larger with that many pages but its not too much, this site probably has nearly that many pages and all of them are visible. other than use of a little more memory to cache the site map, the number of pages does not impact performance at all.
users will only see the pages that correspond to their roles so the navigation experience should not be difficult for the users.
for roles, its ok to have lots of them, but no user should be in more than a few roles, user roles are stored in an encrypted cookie and a cookie can only hold a small amount of data, so if you add a user to lots of roles it will be truncated in the cookie and he would not get all of the roles into the cookie. the only potential problem with lots of roles is back end management of roles might become more difficult
the performance of individual pages is based mainly on how many db hits are needed to generate the page, which is why you want to avoid loading anything that the user is not going to see
performance will mainly be impacted by how many simultaneous users/requests and how much available server resources to process requests and how expensive is each individual request to process, ie how many db hits, how efficient those db hits are