Is it possible to audit log only specific users (thus keeping the Audit-log under control and not harming performance)?
Background Sitecolletion admins can view and edit everything (de facto SP functionality). Even strictly confidential documents (on which unique and limited permissions are set). Management is not happy with that. They want to track/audit those highly confidential documents and especially the site collection admins*. (Those 'Top-secret' documents are scattered throughout the sitecollection)
If this is what you are facing then you should probably do a comprehensive security review and lock down the permissions on the site(s) in question, removing all but one or two trusted people from the role of Site Collection Administrator and then creating new permissions for other users, either as a different security group, a different permission level or (better option) both. This will get around the Site Collection Administrator 'full access' issue.
If you have your heart set on monitoring activity then you can get started simply enabling auditing in your Site Collection Settings. You can then slice the data as you need but there is no way to audit just certain users. Performance with auditing enabled is not a significant concern unless you have a site with hundreds of thousands of hits per day or a dramatically underpowered server.