[JohnWoo]

LEFT JOIN don't only add server load (besides it don't add too much - you can try to remove that lLEFT JOIN and look yourself at timings with debug=1)
If you have large forum with many different users and user groups and with many forums invisible for some of that usergroups or users, that LEFT JOIN make fulltext search run faster (and sometimes much faster) because some (or many) posts will be excluded from MATCH => AGAINST (and that part of request takes the most part or server load)
Second reson is limiting mumber or results returned by search. vb2 version of fulltext search first do lookup in ALL posts with limiting number of matches and after it cut (from that already limited number) more posts looking at user permissions. So, I don't think that vb2 fulltext search works as it must work - sorry