Same deal. Pressure bearing items are manufactured under strict control. You might get away with it, but you might be making the cause of a lawsuit. Maybe.
Cast materials are prone to porosities, inclusions, and other flaws. Cast plumbing items are scrtinized quite heavily in the manufacturing process compared to forged or machined components. The processes Shapeways are using to produce metal products are (as far as I've been lead to believe) NOT subject to such scrutiny. Generally speaking, these parts are intended to be "ornamental" in application, even if practical in design.
For prototyping purposes, having a proper housing made should be reasonably inexpensive, as machining from bar stock should be fairly simple. No, it's not going to be as light and pretty as the final product, but it should give valid performance data otherwise. As I said previously, the impeller should be fine so long as it gets properly balanced. (Actually, 3D printing should be the cheapest way, short of mass manufacture, to make a turbine impeller.)
I'm not saying it can't be done. If you do it, be sure to get the part examined and tested thoroughly prior to practical use.
Hmm... in re-reading your post, perhaps you meant the DMLS process, as is used for making titanium parts? This may be a better alternative, but I'd still advise extreme caution and rigorous testing. Contact those who manufacture using this process and ask their advice on suitability of their technology in your application. Your mileage may vary.