A well-sorted example of the "headers look good, but the results are disappointing": all the 1,000+ HP LSX engines out there using the original truck manifolds.
Same is true for up to about 700 HP on the Toyota 183" 2JZ engine: a cast manifold is really close to the hand-fab $2,500 header, and lasts 10 X as long. Mine is made by SPA in Brazil.



What else favors cast instead of tube?
1. much lower underhood temperature
2. less lost heat energy reaching the turbine inlet