I tried a Carradice cape for a while, I didn't get on with it, seemed to slow me down even without a headwind and made strong side winds scary. I also sometimes ride with someone who swears by his, maybe it's something you have to try...
I'm firmly in the camp that believes staying warm is more important than staying dry. I was out in the rain all day yesterday, it wasn't till I got home that I realised how wet I was, I never felt uncomfortable all day. Breathable materials have their limits, IME well placed pit zips do more to aid ventilation than the material. It also helps to not have more than is needed under the waterproof layer, I'm always puzzled when people put a coat on over the jacket when it starts raining, I always swap one for the other.