Assuming it's just a port with no obstruction, such as a valve stem, the potential increase in airflow is proportional to the increase in the area, which is the ratio of the diameters, squared.... So in your example, (0.404 / 0.335) ^2 = 1.206 x 1.206 = 1.454, or a 45% increase in flow at whatever the pressure you are working at.... If there is a valve stem in the way, for example inside the throat of a valve, then that area must be deducted from the throat area before making the percentage calculation....

Although there is no hard and fast rule governing the FPE increase from such a port increase, my experience has been that up until the point where the port diameter equals the caliber, and for a PCP shooting at the maximum possible, the FPE increase is proportional to the port diameter (not the area).... Again, this assumes that the other factors (pressure, barrel length, caliber) remain constant.... Therefore, using your example, you could expect a maximum potential increase in FPE of about 20%, which for a given bullet weight should mean a velocity increase of roughly 10%....

HTHs....

Bob