HEy John,
Yes, Yew actually is pretty robust, even with no sapwood. When I was living in Oregon I had a roommate who made a yew bow and he removed all the sapwood. It was rather short, and I remember him overdrawing the hell out of it. It had to be about 55 inches long, and he was drawing it 28 inches! And it wasn't backed with anything. I used to cringe every time he shot it, just waiting for it to explode. It never did.