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.