I'm not convinced it is boiling since there is no indication that there is cavitation in the pump. Unlike the E46, the E90 has a PS oil cooler. It's probably a slight expansion in volume due to temp, and an increase in pressure due to sustained high RPM and high temp.
The E46 has worse PS issues unless you change fluids to something that reduces cavitation (like redline ATF). The feeling of cavitation is that the feel of the wheel changes with RPM, an example is the turn in after braking after a high rpm burst. Hell the leaking PS hoses on the E46 were considered "normal" by BMW.
The steering system performance has been rock solid on my E90 so far. It's a superior design to the E46.