Julian,
I've always wondered if the cotton bags are a bit like the old cotton tents -- waterproof until you touch them, the surface tension is altered, and the water comes through? I'm guessing the wax works to prevent wetting-out? At any rate, you sound like the fellow to ask, and it would be nice to put this question aside. Certainly, the cotton Carradice bags have a wonderful, classic look that really can't be matched in modern materials. Nothing says "expedition" quite like a Carradice, at least to my mind.
Best,
Dan.