Actually the cooling system DOES NOT run at 2 bars. The venting valve of the expansion tank is set to open at 2 bars, which is overkill, and effectively will allow for plastic components to rupture abruptly, loosing lots of coolant in the process. The 1.4 bar venting valve is OK. I use a 1.2 venting valve on the e39, and I haven't had any issues. That car was also designed with the 2 bar venting cap. During the last year I added less than 1 oz of coolant.
Lowering the venting pressure, will allow to loose some coolant through the cap - but you have to be careful. the plastic components probably won't break, but you have to monitor, so you don't loose too much coolant and overheat (in case something goes wrong).
Also, lowering the venting pressure, you have to make sure your coolant level is not higher - it will build up more pressure and you will start venting if the temps go high. If the coolant level is lower, it allows for more expansion of the coolant, but this is also not desired.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.