Depends on the tune, but it also adds lag as there is substantially more volume to pressurize so it may move the peak hp and torque to a less desirable point higher in the rpm range making it less likely you'll have it on tap.
I just got my 2017 WRX tuned. The bottom line, the FMIC is good if you are want to have consistent power at around 4500 and over RPM (assuming you are going to keep the RPM at that level all the time). I might install one down the line but for now, I think I will keep it stock. Why? I'm not running hard on my car let alone racing, drag, drift, etc.... My goals were to have a nice fast, efficient and reliable Stage 2 car. BTW, if you do still gonna go with FMIC. My tuner recommended ETS first choice and Cobb as a second. He mentioned that ETS has a better pipe routing.