The load of a COUNT-Statement is far too high on busy servers.
It will delay your forumshome by several seconds (!) and put an enormous load on the poor database.
If your fora are quite busy (>100 ppl online), I would disencourage you of using this hack.
Unfortunately I am no sql Guru. There is for sure a way to form a statement less ressource intensive but I am afraid I can only critizise not help

Maybe anyone can jump in here?
BTW: Allthough I may sound a bit harsh here (which I am not, I hope), I really appreciate everyone releasing hacks and I know it's sometimes really a mess to deal with, so please don't feel offended