I sure hope it's that simple! Also, FWIW, I tend to try to give all my stream files really short filenames, because going through and applying language codes to each of them in tsMuxerGUI is a big PITA if the language-identifying part of the filename gets truncated in the display.
Oh, and one more thing that might be a factor--I mux graphical BD-SUP files, not SRT files. It's possible tsMuxerGUI has some sort of limit on its SRT-to-SUP converter too.
Yeah, the 8Mbps limit is a real nasty thing once you get close to it. Audio tends to be CBR so you can make pretty safe assumptions about that, but subtitles are all over the place.
The only way to test it is to burn the potential coaster and watch the whole thing beginning to end. If the player goes crazy at any point during the movie, you went over, but you don't know by how much, so it's trial and error. I personally would cut lossless audio first (replace it with lossy), replace any high-bitrate lossy tracks with more typical bitrates, then start cutting alternate audio tracks altogether. I don't cut subtitles