I have not measured it. Maybe somebody else here has. The stopping power, dry, is definitely not as good as on alloy rims, but I'm mostly comfortable. In a total emergency, would the difference be crucial? I can't say for certain, but if you are very risk averse, then it's a consideration.

When wet, the rim brakes on carbon wheels scare the bejeesus out of me. Not safe, in my book. And one of the key reasons I wanted a disc brake road bike.

I do ride in the rain. Also, in the midwest, you may not think you are going to ride in the rain and then you are surprised. Having lived in southern CA, i know that doesn't happen there. If rain is coming, you know it well in advance.

An alternative is to buy carbon wheels with an alloy rim track, like HED jets. If I remember reading it right, I think HED basically said that they couldn't stand behind the safety of the carbon wheel/rim brake combination and so wouldn't sell them. Maybe somebody here knows more about that.