This link shows tests carried out on breathable fabrics. It demonstrates that Paramo (Fur type Technology) is more breathable than eVent which is more breathable than Gore-Tex!
This experiment shows what force is required for air molecules to pass through the fabrics, which is a good indication of conventional breathability. However, it does not take into account any of the thermodynamic effects and real world problems that lead to condensation inside waterproofs.
The report also suggests that feather and fur type garments are too warm. We would recommend wearing less beneath them and utilising the integral vents. This gives them a far greater temperature flexability than conventional layering systems with less time consuming layer changes! Wear them just like a softshell, which happens to keep you dry. Here's some recommendations for different activities.