If there's a poll somewhere, I would think the split into 3 groups makes the most sense.
I never thought about how the mix of bikes in one forum actually contributes to the overall forum but I can see that it does and would vote against separating the forum into brands.
I can say that I do think it sucks trying to search a MegaThread to find info as there are tons of shit to sort through but at least I know its in there somewhere!
If the main forums were split into categories instead of bikes, I think that would be better:
Instead of a Bikes Section separated by types, have the Section split up by Engines, Suspension, Electrical, etc and THEN have the bikes section as a subforum from those so when folks are wanting specific info like fork swaps for a DR650, the search results from that section would be nothing but suspension related thread as opposed to a search now from a MegaThread that shows a damn post everytime someone mentions the search term and may or may not a valid return.
OR, vise versa...split the bikes section further into engines, electrical, etc... Once you enter the Section: Thumpers, you get a long list of bike megathreads...searching for specific topics IS difficult and could be remedied by breaking out the subjects folks are searching for (engines, suspension, etc...)
Philosophy is written in that great book which ever lies before our eyes... Galileo