With that setup, the best way to constantly hit 16 PSI is to go and get a protune done. Having that aftermarket IC, you arent getting anything from it since the AP doesnt account for it in its map. And if you go and get a protune, think about getting some type of 3 port BCS (i.e the grimmspeed unit). Im running a 20G with all the supporting mods (check my profile) and when its colder (denser air) I tend to hit above my target boost, maybe only .5-1.5 PSI depending on how Im driving.
Even when I was Cobb stage 2 (just a TBE and Cobb SRI) I would occassionally hit more boost when it was colder out, but not all the time. In short, it happens, but if you want a more consistent car, get a protune.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX