Hello Karlos,

Unless one actually tests the flow of an intake manifold on a cylinder head you don't really know.
The harder (more expensive) way is switching inlet manifolds and testing on a rolling road.
A factory unmodified inlet will not support the airflow requirements of a modified head.
The factory manifolds may be modified to produce good results.
However, nearly all of the big HP guys have custom inlet manifolds - or at least modified factory inlets.

The inlet is far more important that a tubular exhaust manifold - up to 300 HP.

Just my opinion