On my old Di the boost limit was 16psi, yes not via the MAP sensor as you say, just thats when the stupid thing would just a have hissy fit and go into limp mode, at 16psi, or 14psi if I wound in the needle valve for faster spool up. After changing out the MAF housing for a high flow housing, no limp, no hissy fits, nothing, I wound the boost out to 30psi and still nothing. Drove great.