IMVHO this sounds like a good idea. I liked it when the whole forum front page fitted on the screen without a scroll bar. Splitting it into sub-groups should help people find the information/discussion they are interested in more quickly - especially those who, unlike me, do not enjoy browsing through but just want specific info fast!
I guess, if you ever went down the route of selling advertising space on the forum pages, it would enable advertisers to reach their target market more specifically, too!