Yes, I found that adding video_threaded=true improves performance. Also rewind_enable = false, can also improve performance, and enabling it cause some cores to segfault.
Also setting sound rate to 44100 also improves audio performance in some cores.
On overclocking, 950 is what I have, running smoothly all cores, except some x32 games (virtua fighter and some others).
Also, I’ve read that using a lower ram split, leaving at least 192 mb of ram for the cpu may be better, at least that’s what the developer of muppen64plus-pi says.