I would say 15.5 is safe for the stock turbo with a stock uppipe i wouldnt run over 15.5 on stock upp because you will melt the cat in the turbo, then you wont be about to shift pass 3k
Safc is the fuel controller, Avcr is the boost controller
And sorry apexi doesnt make a downpipe or a uppipe for a wrx
Goodluck if you get one