This
may be helpful: Much of the Bosch Motronic 7.xxx is documented and defined. Mostly it is a matter of figuring out what you are looking for. I expect the earlier Boxsters with ME5.xxx are similarly written.
Nefmoto has some good documents and translations:
You will need to register and login to see or download any of the files.
S4 Wiki has some other good basic Motronic info.
https://s4wiki.com/wiki/Tuning