[kmike]

There's nothing wrong with your approach, but as orban already pointed out, there're too many places you have to track to keep the post positions updated, and renumbering the posts on every delete/merge/split is a costly operation. Also, you'd have to find the last postpos in a thread for every new post, which will be far more frequent operation if your forum is active.
Personally, I wouldn't bother with this approach.