So I got a email back from GFB. The sensitivity setting on the controller does affect how quick DC is changed to compensate for boost taper.
Therefore if I set a peak boost of 25psi on 75 DC, it will adjust DC potentially to 100 when the boost starts tapering off.
Gain is slightly different and I quote "The Gain on the other hand does not really affect duty as such, not linearly anyway. What it does do though is it locks the solenoid shut until it reaches the boost level associated to the Gain setting. So if you’re wanting to run 20psi boost for example, and you have the Gain set to 50%, this means the solenoid will stay closed until it reaches 10psi. If you wanted to run 20psi and have a Gain of 75%, the solenoid stays shut till 15psi."