Well partially 3D printed anyway. My employer is now doing custom tube bending and cutting and we worked with TFS to build these custom headers for a show. That collector was actually printed in stainless using DMLS, Direct Metal Laser Sintering.
If you can CAD it the tubes can be bent and cut to your specs. It still has to be all welded together though. We’re not quiet to the point of 3D printing the whole thing but the tools and processes are improving all of the time.If ordering tube headers was as easy as ordering 3D printed parts, I'd be tempted to CAD up a turbo header setup.