So taking into the fact that for approximately every 1 PSI in pressure increase the boiling point of water is increased by approximately 3 degrees and the fact that most coolant increases the boiling point of the cooling system.
Why in gods name, even at altitude would you need to pressurize the cooling system to close to 30 PSI and increasing the boiling point of the coolant approximately 90F!
The E46 cooling system really operates on a cushion of air inside the expansion tank anyway.
Something just does not make much sense regarding a 2 Bar coolant cap.
I sure would not want my cooling system with all the plastic and rubber parts operating at 30 PSI and over 200F!
Do all the math you want, put a pressure gauge on any E46 and drive it during the Summer months and I would bet you would never be much above 10-12 PSI at its hottest operation point.