Just like Len said.. The port can't support the valve without welding at anything above the 44mm valve size range.. Using a valve with more potential than the port that can feed it, does nothing... Except add valve train mass, require more spring to control, add friction and other mechanical losses and REDUCE power.
Here is a proper port to support a 48mm valve with shape, volume and velocity all taken into consideration.
The result was 284HP on pump gas, normally aspirated and at less than 6,500 RPM.
Click to view attachmentAnd now a port optimized for a 56mm intake valve :-)