I'm wondering if it's maybe that the ceramics cool down faster, whereas the semi-metallics hold the heat longer. I'm thinking the heat possibly evaporates the water from the pad surface longer?

Again, this problem appears completely temperature dependent. I can drive in a driving downpour for an hour when it's warm out and the braking action is immediate. It's only when it's wet AND cold out that it occurs.

Also, same roads/same conditions-- I never had it happen with my 325i with semi-metallics.