owch... I don't know if this is a limitation / problem with the s-afc2, but other possibilities could be your fuel system is maxing out and no more fuel can be added. I've never had to add as much as 20% fuel in the s-afc, but increasing fuel pressure at your FPR will richen your overall fuel curve so you don't have to add as much fuel in the s-afc.
If you boost fuel pressure across the board the ecu is going to take it back out. It's possible you are maxing out the air flow signal with that much increase. The maf maxes out around 550 rwhp, so if you are close to that level and the signal is close to 5 volts, adding 20% might be too much.
you mentioned in your other thread that it does not go lean at the end...im guessing you mean its fine at redline. what ECU are you running, OBD1 or OBD2 on your swap??? if you are on an OBD1 ECU perhaps its the lean spot from 4500rpm ~5600RPM that others have also had on the stock fuel system. this problem seems to inherent on OBD1s running on stock fuel at higher boost levels.