Highest safe or the highest usable without de-tuning ?

You could de-tune a 12:5:1 motor on 93 oct and not have any detonation issues, however the motor will be screaming for better fuel then 93 oct.

If your looking for a good all around compression ratio for a 93 oct bseries setup I would recommend 11.5:1-12.0:1 with an 81-82mm bore