I have a Sugoi Helium jacket, and IMO it breathes really well for a rain/wind shell. It's not a current model, though, and the one I have isn't convertible. But I'd assume that the current Sugoi wind/rain shell would be about as good as the Helium, if not better.

I'm not sure offhand if Sugoi makes convertible shells, though.

And I'm too lazy to look.