I also ordered one of the 1.2 bar caps from German Auto and can't wait to get it in my car. But, remembering some of my college chemistry, I don't think it will result in increased overheating. It would probably increase the likelihood of overboiling, i.e. coolant out of the radiator cap due to the lower pressure. But, the coolant would still have to reach a certain temperature before that could happen. In this case, the 1.2 bar cap is still above 1.0 bar or sea level pressure and would work (like the 2.0 bar cap) to prevent overboiling.
Also, the point of antifreeze/coolant is to both lower the freezing temperature and increase the boiling temperature. It just seems like the new 1.2 bar cap will work. Still, there must be a reason BMW is using a 2.0 bar cap.
I will update my experience once I have replaced my current cap with the one from German Auto.
Just looking to have some fun while enjoying the ride.
'01 540i 6-Speed