In the power game a custom-designed and tuned exhaust manifold to facilitate smooth and unrestricted exhaust flow will clearly be beneficial.
The standard E15ET exhaust manifold is a well know restrictive area and is always most often than not replaced when chasing big HP out of the E15ET engine.
A common problem with the exhaust manifolds on these cars is melting of the divider flange in the middle of the manifold outlet.. This can prove to be a performance reducing factor as it literally blocks 1/2 the engines gas flow. New dividers can be re-welded in to the existing manifold or the flange can be die-grinded back far enough to prevent any restriction.