This would only refer to if someone posted a link inside a post? If someone is coming externally, i.e. via google search, link on 3rd party website no matter what you will need to do the 301. Once they arrive to the forum when navigating you won't have any 301 issues, right? just curious.
The overhead for the 301 script that Kier wrote is very low. It is doing a select statement on a hashed index. You can't do a simpler / faster query if you tried. The re-direct part does add an extra layer, but for the user they should never see the difference because of how fast the redirect will happen. 0 bytes are transmitted to the user before they are sent to the destination page. There shouldn't be any delays. Unless of course I haven't a clue as to what I am talking about, which is always a possibility.