Comment by onre

Intake and exhaust manifold shapes affect the power and torque characteristics of the engine. Also, these are not cheap parts. Also a quick glance at the parts catalogue suggests that the example engine, BMW B48, has different pistons and connecting rods for different tune levels, suggesting that the cast engine block is one of the few components which are actually interchangeable between the versions. Didn't check, but I'd guess the turbocharger compressor and turbine wheels might be different, too.