The issue I have with the displacement split (logical as it seems) is that wherever you make that split, you will end up with two general Thumper forums. I would hate to miss out on a lot of the discussions in the smaller group because I may spend more time in the bigger group. Ideally, generalized discussion should be in one place.
My personal feeling is that whatever you put in a subforum, it should be pretty specific. The DR650 thread has 75K posts, XRL has over 50K, WR-R over 35K, and even the Husky 610 has 25K posts. In five years on ADV, I haven't even looked at some of those threads. And those are just the single threads! ( The KLR and F650 threads are actually much smaller, presumably due F650.com and KLR650.net, etc.) Those are massive numbers of posts, but I would wager most inmates that don't have one of those bikes have spent little or no time in those threads. So turning those into subforums doesn't detract from the general forum.
Having bike specific, not make specific, subforums would turn the megathreads into their own subforums, and hopefully make it a lot easier to search for info. But this would only be done for the bikes that need it.