If you are trying to hit a certain horsepower target, then you must get rid of that boat anchor of an OEM exhaust manifold. Say for instance you got your STI and you're tired of Evo's picking on you at the drag strip and you want to hit around 350 wheel, then you would need an exhaust manifold that will be able to flow the power it takes to hit those numbers.
RallySport Direct carries many major brands such as HKS, Invidia, and Killer B Motorsport. We carry both equal and unequal, and for those of you looking to make big power, PERRIN and Tomei both make large piping manifolds for those of you who need to flow some serious power.
And if you don't drive a Subaru, then cp-e makes an exhaust manifold for the MazdaSpeed 3. This manifold comes with an external wastegate provision as well. \
Remember when replacing exhaust components, turbocharger components, that you use new exhaust gaskets and hardware to make for a clean installation, but more importantly, so you don't double your work.