Video bitrate won't change anything--the 8Mbps is reserved for non-video streams, so it can only be adjusted via audio and subtitle changes.

192k audio streams (most dubs and alternate tracks) are already pretty dang tiny--if audio is where we want to make cuts, we need to cut down the big ones, the lossless tracks. Even encoding with a slightly smaller lossy core isn't going to save much space--switching from lossless to lossy on one or two tracks will free up a world of space. The question is: which tracks? I think hairy_hen's 5.1 track should be sacred, but that's about it.