how much timing advance is good for a '87 t2 w/91 block and turbo, streetport, 3mm seals, full exaust and intake goodies, 10psi boost max?

Does adding more boost like 12 or thirteen need different timing?

 

If you are running the stock computer I wouldn't change it. Not after the port job. If you do experiment only go a few degrees advance.

The technical answer to your second question is yes, if you are using a standalone efi. For stock computers just leave it alone you can't adjust anything individually just as a whole.