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.

It looks so 80's, lol. I'm gonna order the 1.4 bar E30 cap I posted, I have a few parts to gather from the local dealer (oil filter cap, windshield upper molding, strut dust cap) so I'll report in a few days when she is in and installed.

So Doru, effectively replacing with the 1.2/1.4 venting cap will allow for more coolant loss (as that pressure might be reached more frequently) but should avoid ruptures to an extent because the venting pressure is brought down?

Basically the cooling system is going to vent whether you like it or not, and would you rather have 30 psi or 18/20 when that happens?

Yup, I am going with the 1.4 as well, the one you found. I have a huge list of parts that I'm ordering. Going through Craig at Open Road BMW. You can't fine a better price. 15% over cost. Let me know if you want me to post up his info.

To reach 1.2 bar DIFFERENTIAL pressure, the coolant should be above 126°C - that's in an M54B30 engine. That's pretty high.
You should NOT loose any coolant with a 1.2bar or 1.4 bar cap. If you loose coolant, it means your coolant went above 126°C (1.2 bar cap), which in turn is a different problem (stuck t-stat comes to mind)
The 1.2 bar cap I purchased from GAS. here is a short reading about the pros of 1.2 bar cap

And here is a debate about the subject (1.2/1.4 bar vs 2 bar cap). This will shed some light whether to go this route or not.

I read that entire thread and a few others over the past few days. ahahah, funny you found that.

If you haven't already done so, use the BMW ETK (on realoem.com) for your car and VIN number to be sure what your X5 should have. One alternative to suggest, the factory pressure cap on my 2002 X5 3.0 diesel, it's rated at 1.4 bar. Part number is 17 11 1 742 232.

Note that later BMW E30 (facelift from memory) had a specific low pressure cap as part of a US market recall to minimize blown heater cores and the resulting scalding / burning a driver's right shin from the gush of hot coolant. Research this a bit, but from memory it was rated under 1 bar.

Aha! Awesome find guys! The theory sounds plausible and will be getting a lower pressure cap as well.

Been thinking of the cooling system because though I have replaced all my cooling system hoses, I just realized I have a nest of heater hoses I havent replaced yet.

Doru, thanks for translating the vent pressure to a coolant temp needed to vent.

I havd a habit of monitoring my OBC coolant temp and driving I get up to around 92 and spike to 96 at the most briefly. Engine off I've never seen it reach 100.

What temps are you guys seeing in your OBC's?

If you look at post 7, that is the same part number as the one you provided. You sure that is a 1 bar cap.