I thought I'd add a quote from Vizzard:
So how do you decide whether a port is too big or too small?
DV:- Well you have to understand that as a head porter you could be approaching this situation from two different vantage points. First the head porter may be called upon to produce a design for a spec of engine where certain aspects are already decided on. Let’s say we are going to do a hot street head design where the valve lift is limited by the need for long term reliability. Here the port should not be so big that it continues to deliver an increase in flow significantly past the point of maximum valve lift. If it does than you have just designed a port for a valve train which the engine is not using.
In the second situation the head porter could be called upon to port a head for the maximum power output possible. Here the head is made to flow, consistent with having good port velocity, as much as possible. When that’s done the engine builder then has to come up with the appropriate parts that compliment that cylinder heads characteristics. This is especially important in the cam department.
In the second instance it is really important to keep an eye on the flow increases made versus the port volume increases. When flow only goes up marginally for a given volume of material taken out then the port is probably already too big. However making that statement is close to oversimplifying the situation. At the end of the day it is vital that the port be velocity mapped to establish where the port is hyper active and where it is lazy. Again though this is simplified we can say that the lazy areas need to be filled in and the active areas need to be enlarged.
At the end of the day there is a certain port area that best suits a valve diameter and that this port area is dependent on the down draft angle of the port and just how good the porter is at getting air to go around a corner that normally, it has no desire to do.
So you can optimize a head for a cam and intake or you can increase an flow and find a cam and intake to match.
I don't see 18.9 cfm increase @ 0.35 inch lift = 20-30 hp.