Hi Gearpins,
The QRH for Dual Engine failure recommends flying at speeds which seem closer to green dot for maximum glide angle rather than for a high ROD at higher speeds. You are not attempting to get down to 10,000 feet rapidly - but to give yourself more time to resove the problem. I guess once you have lost all cabin differential pressure, then the RAM air can be opened and would aid cabin ventilation and FL150 was considered to be a good compromise.