The design with the welded tube actually won't produce much more turbulence than a smooth section tube. The key is consistent diameter and no internal obstructions. From the photos you can tell he's using a TIG process and I'm sure a good inside finishing. In fact, that type of tube construction is used commonly in professional motorsport and aviation. I've seen plenty of tangential exhaust systems made like that.

Yes, hydroforming or some sort of composite would be better, but that's a helluva nice job and some good ingenuity. Plus, it appears he's got the dyno paper to prove it. Oh, and parts in stock!