If your lower fuel correction below 40 g/s on sti ecu, i dont know what load columns are on wrx, are off and your higher rpms are off in same direction then i would adjust injector size to correct across board. if its off in higher rpm but close to +-5% in less than 40 g/s then i would adjust maf.
on my sti i adjusted both the injector size from 503 to 513 and rescaled part of the maf on my stock intake and my lower fuel trims were less than 4% and high rpm was almost dead on.
so the answer to your question depends on how it is off and where.