Rocael,
have you seen Don's scalability article? I'm not sure what the most efficient way to check permissions is after Don's changes. The joins you are making won't always yield correct results though. It might be you should use the acs_object_party_privilege_map view. Are you using Don's permissions enhancement (they are on the 4.6 branch)? You probably should be.
I think if in certain cases even the new enhanced permissions API doesn't offer good response times we should consider simpler solutions. For example in the Bookmarks module we are using the permission API only to indicate that certain bookmarks are private and that is obviously an overkill (a private_p flag would be sufficient). I can see how permissions might be useful for WP presentations though.
