The benefit of a header over the stock OEM exhaust manifold has to do with how well the header flows and how well the stock manifold flows.

Toyota gives a good example of this with their old 4 cylinder truck engines. The carbureted 22R engine came with a lousy solid iron manifold. Just about anything was an improvement. The 22RE on the other hand came with a factory Tri-Y header. Most headers were no improvement, and in fact some were worse than the OEM exhaust.

The 3.4 Toyota has a pretty darn good exhaust manifold. It not easy to beat it with a header, particularly a ho-hum header. For the money, there are better places to go looking for performance increases. Like the stock muffler if you're running it.