In reply to David Staples:
If it were me I'd being going for a waterproof that fits, in your case a medium, and wearing the down jacket over the top as a belay jacket. If its too wet for a down jacket, then it probably isn't cold enough really. If you try and get a waterproof that goes over the down you'll compress the down anyway, making it less warm, no matter what size you go for. You could go for a monster size to try and avoid this, but it will be mahoosive and unwearable under any other circumstances.
If you are worried about down in the wet, consider investing in some primaloft too, as it works better in marginal conditions and still lofts when damp. I'm assuming that you are wearing the down on belays etc? If you are wearing it when walking in then you probably need to take a look at your layering system!
