Breathability has nothing to do with the seams Mel. It's the waterproof membrane that makes it breathable. Your jacket doesn't have a waterproof membrane, but it will be made of nylon or polyester, with a waterproof PU coating.
I have a Peterstorm pac a mac and it's poorly made. Seam taping isn't very good and started to delaminate early on, and its only good for light, short rain showers.