Though I've not personally used this, it is used extensively by several other people I know. A lot run highly modified turbocharged cars, some off-road vehicles, and the like. It works very well. It will not boil over, even at 300*F temp. You do not need to run a pressurized cap with it - just a cap without a pressure seal is fine, or no cap at all.

Though it is expensive, it does work, and I'll be using it when I put the new radiator into my project franken-Volvo this summer.

Hadn't thought about using it in the BMW - but...it does make sense...since our plastic bits are so prone to explode after time. The lack of 20psi of pressure inside the system would probably extend their life almost indefinitely.

It's good stuff. Been around for years. It's an additional layer of protection.

One thing to consider is that pressure isn't the only factor with the plastic components, the PH level or the coolant and temperature are what break down the plastic over time. I can't tell you how many plastic hose ends I've seen at work turning brown and feeling like wet cardboard on the inside that weren't leaking yet. That affect was from years of heating and cooling with the acidity of the coolant wicking the oils out of the plastic etc.