You are right it would be hard to quantify, mostly just asking for experience. Even if the cape tends to be no wider than the riders body, it is still a full sheet being stretched between the head down to the arms like a sail. That is going to put more resistance there. Also to get a little technical, it will increase the amount of sidewinder you catch while riding too. The wind no longer cuts between your arms and torso, it gets caught in the cape.

So I still stand by the question, of the people who have used the capes, how did it affect your ride? Does it make it plausible or even pleasant to bike in the rain? Did it turn you into a sail that the wind pushes around?