Let's say 50% ms-frames and 50% ss-frames, how much more compression would this yield than 100% ss-frames?

People are always talking about how many bits j/s-mode saves, but how many are we talking really... 10%? 25%? 50%?

Note: I trust lame's j/s-implementation, so I would use it even if the savings were minimal, but I would still like to know out of curiosity.

For pure mono, 50%.

Hi. I experimented briefly on this with classic rock music back with Lame 3.87 and 3.88.

Back then it was more like 80%Mj and 20% Simple stereo for this type of music. The joint stereo thresholds were not adjustable until later.

When simple stereo was used exclusively, the bitrate of the file would increase around 15 to 20 kbps but this would be very dependant on the music involved.

A simple test with your preferred music should help. I would expect very little change with preset extreme or insane as only perceived mono is encoded in mj anyway. Preset standard might show a significant difference.