STOP! Don't even think about using shifter housing for a brake. It is not radially supported and can literally split apart under braking forces. You couldn't even fit a brake cable into it anyway but I'm sure that hasn't stopped someone from simply using a derailler cable instead.
With that said, I'm currently using compressionless
brake housing (by Jagwire:
http://www.universalcycles.com/shopp...&category=4384) specifically intended for disc brakes. Honestly, if I hadn't been having problems with my set up (ended up just being a headset adjustment) I'd have never bought it. I can't say my brake feels anyway better with it though there may be some slight benefit.
The biggest benefit to disc brake performance that I've found is performing as accurate of a set up as possible allowing you to run both pads as close as possible to the rotor. You may need to put up with a slight amount of rubbing to get there as no rotor is perfectly flat and truing one to perfect flatness is not something I've been too successful with.