Looks like -5 is the sweet spot here, at least for vorbis set I have.
Input 90.419.200 Bytes
Setting --jobs=6 -b e
Code:
-1
Clock Time : 2.907s
Output 81.047.039 Bytes
-5
Clock Time : 25.681s
Output 79.436.915 Bytes
-9
Clock Time : 109.501s
Output 79.430.984 Bytes