The big problem with headers is that the pipes are so much larger than the stock manifold. Even if the diameter is smaller, the total volume is larger. That means the engine has to work harder to fill them. That also means low end power suffers. Headers DO work at very high RPM's. They were designed for the JDM STi engine which has a higher redline than the US model, so on that car the engine's usable power range is such that it can benefit from the headers. On a US car, the headers finally start making power just before redline. Unless you have an extensively built engine, you would be better off spending your money elsewhere.