Not an expert, but i think it really depends on how the DWR is applied, how much, what kind, etc, etc.
DWR's can potentially decrease breathability. Something like a DIY Nikwax water soak won't really affect it, but some factory kinds can.
Two of the the most common ways that factories make lightweight, thin fabrics more down proof, for example, is by calendering it (putting it through high pressure rollers that are heated) and applying a heavier/thicker DWR coating. The more down proof a fabric, the less breathable it will tend to be.
I'm not sure about the various sprays and the like, as i've haven't used these much. But i would think if you over spray, you can decrease the breathability some.
Hopefully someone with more concrete knowledge and experience with different DWR's will chime in.