I agree on the whole, sub-forums sound ideal, though I also think a single SM sub-forum would get messy again pretty quickly.
The obvious answer is one Codex, one Forum, rather than trying to do it by Chapters I think. Have Salamanders and White Scars and Home-Made included with UMs and Fists, but with BA, SW etc in different forums. After all, the codex 'is' the list really, and people building lists from a particular codex are gonna share certain concerns, problems etc.
It also means that unecessary info is kept to a minimum - if you don't play DA, you don't have to trawl past endless Deathwing posts.
It would be easy enough to update if GW decides to change things around, I would think. I know you've included Daemons as a seperate army, but should they release a codex for another army-type that doesn't yet exist, I assume it wouldn't be a problem to add another sub-forum, or even fold one back in if they (I don't know) drop Space Wolves for instance (sorry Firewolf if you're reading this... divvent tek me ears!)
Also, it might make people think a bit more about the info they're giving. Sometimes SM players don't post the Codex they're using... there you are giving them advice based on Codex: SM and it turns out they're using BA.
Don't know how much extra work it would all be from y'all on the Mod Squad, but certainly from a user point of view, that looks to me like the best answer.
Hope this helps in your decision-making...
Edit: you posted while I was typing; I want to make clear I'm not advocating a SM sub-forum with SM, SW, BA, DA, BT sub-sub-forums, just 5 sub-forums for the 5 SM codices. I realise that BT and SW don't get that many posts, but even so, I think the 1 codex, 1 forum formula is the best, from our point of view at least.