venting the WG off of one runner is acceptable, but as John said, far from ideal. I would venture to say that you shouldn't be worried about that if you are running a "street oriented" setup. Only on high boost, huge turbo apps should that be a concern. Cast stainless is VERY nice for turbo manifolds because as John said, they are light, strong, and since its cast, the usual problem with cracking due to expansion is not an issue. They can also be polished to a bright shine too for the pimp in you

Here is a pic of a header made from cast stainless for a Ford Turbo 2.3. This guy sells them for $450