Yes - just don't give the Guests group "No Access" to all fora (you would probably want to do the same for the Bots group)
There have been no reported performance issues that i am aware of with any extension that is in the CDB - however the same cannot always be said about some that are in development/abandoned
Just been looking at this and it would appear that you need to add the
cli.phplanguage file into the
upload_max_filesize- can you post the value of that.