Having read most of the e46fanatics thread and other posts here and elsewhere, I have one question which I haven't seen answered directly - why did the BMW engineers decide on a 2 bar cap in the first place? As noted in previous posts, testing under extreme conditions couldn't get the coolant temp above 126 deg. A 1 bar cap will bring the coolant boiling point up to 130 deg and a 2 bar cap will raise it to 154 deg. The cap has two primary functions - to pressurize the system and to act as the "weakest link" if over-pressurization occurs. So what scenario were the engineers seeing when they decided on a 2 bar cap?
I took a quick look at two ends of the automotive spectrum and found:
Toyota Corolla average 1.2 bar
Ferrari (a couple of models) 0.9 to 1.1 bar
From what I can gather, 1.1 to 1.2 seems to be an industry norm. This seems to be independent of the use of plastic or aluminum radiators. Perhaps someone knows of exceptions? It's doubtful that BMW engines are sufficiently unique to warrant such a departure from industry norms.
BMW has been known for missteps - lifetime tranny fluid (although this might have been driven by marketing), vanos seals, the truly horrible design of the CCV system etc. Could the 2 bar cap be another?