The ECM modulates the wastegate electronically, and it's driven by a lot of factors. If you're in manual mode (to hold the engine speed down), you can get moderate boost (5+ psi) at only 1,500 or so RPM. It'll hold boost to lower figures when engine speed is that low to prevent engine damage, but boost is managed by the computer and is generated in response to many factors...engine speed is one of them, but only one of them. The maximum boost I've seen is in the 15 psi range.
It is interesting to watch the ECM try to stay out of boosting the engine unless you're really pushing it. It'll have the throttle butterfly wide open, even when you're at less than 50% on the gas pedal, to try to get the most out of the 2.4 liters of displacement without pressurizing the system. It seems to quickly ramp open the throttle butterfly as you ease into the gas pedal and, only if naturally aspirated wide open throttle is not enough, vacuum will start to turn negative (indicating pressure). I think it's pretty fascinating to watch the computer manage the engine load and other parameters when trying to stay out of boost (to save fuel, obviously).
In our case, I don't find a lot of difference in fuel economy when driving it in manual mode vs. stepless mode. We average in the low 20s around town, which I'm 100% happy with. The deep gear ratio change just off idle is done while maintaining about 50-60% engine load, which actually keeps efficiency high. It requires more throttle opening (which reduces pumping losses), but the ECM hasn't closed the wastegate on the turbo to pressurize the intake...so economy tends to still be pretty good. There really is not a linear relationship between gas pedal and throttle butterfly opening...which definitely takes some getting used to. I don't like that deep ratio change, but I really do think the car is doing what it can to support good fuel efficiency.