Boost control logic for the 15 WRX is virtually the same as the prior ECUs. The overboost feature is not a specific function of the ECU but of the calibration. The peak boost target in the calibration is 15.9 psig at sea level (as advertised), but can be a little higher due to an IAT comp (in a moderate range can push peak target up to around 17.x psig). The boost limit fuel cut (and P0244 DTC) is effectively set to around 4 psi above the boost target, which would put it around 20-21.x psig. However, this boost limit must be exceeded continuously for over 2 seconds, so you can certainly see peak boost in that range briefly (or even 22.x psig that some have reported) without engaging the limiter. Pretty much any typical WOT pull will result in the period of "overboost" with the factory tune, but the factory calibration will quickly reduce wgdc when boost gets closer to the boost limit.
At higher altitudes, the peak boost target is lower (just as with prior Subarus) but the "overboost" function in the calibration is still present, so you will still "overboost" relative to the lower target (you just generally won't see the same peak boost as you would at sea level).
In our OTS stage 1 map calibrations for the 15 WRX, we get rid of the gimmicky "overboost" calibration and we use a peak boost target of 19 psi (at sea level - progressively lower at more significant altitude). This results in more consistent boost control (and you actually know if you are overboosting or not). Not to mention the many other changes in our stage 1 maps with the end result being greater performance and much better driveability.