I know of a small block engine that had its heads hand ported.
After they started leaking water, they were scrapped, the same model/ type of heads were given a CNC port copied from the same guy who had hand ported the original set..... In other words his program/ parameters were used
The results thus far..... Based on both sets of heads being on the identical engine and dyno, , at the same track, in the same car with no changes other than literally the heads ...It appears there is about a 20 horsepower difference in favor of the hand ported heads...... That was the difference on the dyno...... It appears the track difference is a solid tenth...... The CNC heads are without a doubt " smaller"..... But have more integrity regards the possibility of leaking.
This on a 700 ish horsepower engine