It's always been my findings that headers, whether tubular or log style, generally only break due to poor fabrication or welding process.

If you use stainless steel weld-els (schedule 10 is PLENTY) such as the ones posted from grainger, or www.acestainless.com, you can cobble something together without having it break quite easily.

And a tubular header will for certain have more power potential than a log manifold on a turbo engine in the end, regardless of cylinder count.