Multisite or Access Control Model?

I'm currently doing a proof of concept for Drupal as our Enterprise wide CMS. Our current site is http://www.cdha.nshealth.ca/. The current site is in a complete mess, and lacks and any decent functionality. We currently have 160+ sections on this site with approximately 200 contributors managing their own content.

The access control model in our current CMS is: users have jobs which have roles associated to the job. User A may have a job called orthopaedicPublisher which maps a role to a section of the site (they can only edit content in the orthopaedic section). This model seems to work fairly well for our organization, but i would like to take the site to the next level and Drupal seems like a logical choice.

The only thing i haven't found a clear resolution yet is which approach to use in building the site. I've been at drupal well over a year now and am well seasoned drupal site builder, but most of the sites i've built have had much smaller hierarchy's and only a small handfull of users managing content.

The main requirement for our site revolves around Access Control. Multisite seems nice so that each section could be its own self contained CMS, but seems to add a alot of maintenance to it. I don't want to be managing 100+ databases. I've checked out the Taxonomy Access Control and while it works, seems like it may be a bit messy to configure. I've checked out Domain Access, but we're already a subdomain of a domain (government) and the DNS entries could be a bit of a maintenance mess.

One option could be a combination of Taxonomy Access Control combined with Multisite, but i would like to hear from others what different approaches they may use.