The cap is rated to release at 2 Bar so I definitely would be going above that. In normal service it shouldn't really hit 2 Bar, otherwise you'd be constantly topping up with coolant.

Here's my photos/write up of my pressure tester:

I made this nifty little device, a coolant system pressure tester. I used an old radiator cap & a car tyre valve. I I used a hacksaw to remove the pressure relief valve from the radiator cap. I retained the top portion of the relief valve for two reasons:
- it has a rubber seal so I can still have an air tight seal
- the outer cap still spins independently of the piece with the rubber seal

I did have to separate the outer cap from the pressure relief valve in order to do the mods but I think it turned out really well. I tried it out on the e36 & it picked up a leak straight away. I knew it had a slight leak but I thought it was a lower hose, in fact it was the upper one. It was not a strong enough leak to detect without this piece of kit, you could see an odd drop on the ground after a drive (which is a rare event these days).
Anyway here's some action shots: