The straps don't restrict your breathing at all unless they weren't made right, ie, without an elastic expansion thingie. Since that's one of the easiest parts to make on a pack, it's odd to make a sternum strap that doesn't feature such a basic item. Or if they are tightened wrong. I like them, maybe my body build makes them more needed as others note here.
One thing I've learned in the process of building my own pack is that the only pack that is going to perfectly fit you is a custom made one, made for your exact dimensions. Each bend of the strap curve corresponds to a dimension on your body. If you have a standard body type that fits an average pack design, then you are lucky, if you don't, then you have to look all around, or figure it out, or buy something very expensive like a mchale, or maybe get a custom thing from zimmerbuilt.
I no longer consider mchales expensive, though they are way out of my price range. But not overpriced given the degree of work he puts into getting your fit right from what I gather.