Well...cast manifolds are usually meant for inline setups because a manifold doesn't have to be as complicated and the amount of metal used wouldn't hurt the curb weight as much. Manifolds for other engines such as V8s and boxers require tubular manifolds if they were to go single turbo setups because of length of the manifold. A cast manifold would also require the turbo to be placed by the exhaust ports for ease. While a tubular would allow the turbo to be placed at a more preferred location which could affect numerous other things such as turbo size allowed and the intercooler piping. A upstream catalytic converter can also be placed before the turbo which helps cold start emissions.