Well, I do agree with your facts even if I disagree with your conclusions. But, help me understand: why would a 1 bar cap help anything at all, even if the system does stay <1 bar under normal conditions? By your own logic, since neither a 1 bar or 2 bar cap would vent, nothing has changed overall. I just think that a 1 bar cap will vent, or BMW probably would've put a 1 bar cap on in the first place.
To further compound your thoughts on the radiator in high-demand situations, high rpms means more rpms at the pump, and thus more flow. But, higher rpms over a sustained period almost always means more air flow through the radiator, and thus more cooling/delta T/whatever (and lower system pressure, lower thermal capacity of the coolant, etc). Not particularly relevant with regard to cap pressure, but that's what the thermostat is for--to hold temperature as close to constant as possible to prevent venting from overtemperature or inefficiency from undertemperature.