Hi Marco,
That Cvar was added in to be a workaround for the PS5 shader compiler to reduce the number of Scalar GPRs. This setting only affects pre-shader constant buffers, but you could encounter similar issues inside material parameter collections. However, we currently do not have this workaround available for MPCs, so you must implement that yourself, if required. I hope that clears up your questions, but feel free to reach out again if anything is unclear.