When I open "View posts since last visit" page I got lot of post sorted by posting time. I have some forums what I prefer, and some other what open, maybe, ones in year. This time sorting push me to scroll and look on all posts finding out posts in my prefered forums. This is hard work especially if I don't come one week or more. In that case for me is better solutuion if is possible to change time sorting to sort by forums. Or, even better by forum and time.
Of course, this is my point of view and maybe this is not necessary.
I like your idea, since I too have favorite forums and would prefer to see less of the other posts. On the other hand, I like to use the "posts since last visit" in part so that I don't get stuck always reading posts from only a few forums. I'm wondering if some kind of a weighting system would work.
As an example only, what if you could assign interest values from 1 to 5 for each forum, with default 3, where 1 was a favorite and 5 was least interesting. In deciding how to sort threads, the time since last post would be multiplied by the interest value of the forum. The result puts most of the posts in the preferred forums first, but some of the recent posts in other forums would also show up early, so it would encourage you to occasionally choose different subjects.