I did another re-org, which separates 'Genres' and 'Companies'. I also created some subforums. Please let me know if others would be useful.
I am not particularly fussed about this, but I do have some suggestions you might want to consider.
1. I
think that posts related to companies not listed in the "Companies" section are supposed to go in "Genres" section, right? You could make that more clear by changing the small description under "Companies" from "This category contains Company-specific forums" (which just repeats the information we already have from the forum title) to something like "Use Genres for companies not listed here."
2. Putting each company at the top level takes a lot of room, enough that I don't even see the "Genres" section on the first page on my 1440p monitor. I'd prefer to go back to having a top-level link only for those companies that have several sub-categories, and putting all the rest back under "Other," in order to save space on the top page and slightly improve users' view of the overall forum organisation at first glance.
3. Tandy/Radio Shack and DEC both have about ten times as many posts as the other non-subforum companies, both more than Apple in fact. So it looks to me as if these want to have subforums added. Obviously one doesn't want to go and sort more than 40k threads when the sub-forums are created; perhaps one can work around that by creating a "Not Categorized" sub-forum in each as well, and move all the old messages there. (And maybe sort a few of the more recent threads into the proper categories if one's feeling energetic.)
I can't speak for the DEC stuff, but obvious sub-categories for the TRS-80 forum would be, "Model I/III/IV," "Model 2/16/6000," "Color Computer (6809)," and "Other," the last being for the MC-10, the handhelds (unless they go under Genres), and I don't know what else. I expect that posts Tandy's PC compatibles should go in the IBM PCs/Clones/descendants category; again, a note like I suggested in point 1 could help with that.
I also feel as if it would be good to have a "Japanese microcomputers" genre for MSX, NEC, Fujitsu, Sharp, etc., but I don't know if that's actually justified or it's because I tend to think about those a lot more than other folks around here do.