It is clear that the breathability of your layering system is reduced when you add more layers of less breathable material. However, if you need to keep warm then you will be generating less sweat, so adding windproof insulating layers isn't necessarily a problem - just take them off when you get too warm. This is a system used to good effect by Mark Twight, amongst others, and described in his excellent book "Extreme Alpinism".
Wearing FurTech over a baselayer gives maximum temperature flexibility for variable conditions and heat output, but it can be worn in other ways: wear it over a WindShell (or other shell) as conditions worsen or add an OverShell to it when you stop. FurTech is remarkably effective at drying inner layers, even in the rain, so long as you remain active.