You're quite right - there should be no big problem. Lots of posts here from people who have never shivered at the rear end of the pack in a soaking wet windproof. Based on personal experiences in the past year or so I bought a proper windproof (taped seams, 10,000 mm hydrostatic head) - first big test was Long Mynd Valleys earlier this year in very cold conditions with horizontal sleet, with just a windproof I would not have made it round. Packs into my bumbag no problem and I'll be taking it with me tomorrow.