There are definitely some great porters mentioned, but it's all horses for courses. Many custom porters are going with CNC porting now to save time and make their business more profitable. If I was looking at spending my $$$ with a porting shop, I would be looking at what kind of work comes out of there. At the track, those setting records are using whom?
CNC shops like CFE, MBE, Slawko and Curtis Boggs @ RFD

Old school hand porting? Larry Meaux

Brett's heads have been performing really well, I believe he does the port by hand and has Denis @ Air Flow Development machine them for him.

As has been ported out, most CNC stuff is generic, to meet the requirements of many different customers. In most cases, those seeking more will have them modified afterwards.

Depending on the software used and how the port is digitised will dictate how the surface is generated. I know digitizing off the machine generates a series of points at a series of depths. In something like Mastercam, those points are using to generate a curve, and from those curves a surface is lofted. The surface is copied and offset as a means of tool control. Making changes to the shape requires modifying or re-creating the curve/spline and re-creating the surface. In something like Solidworks it would be easier as it is a parametric modelling software, so dimensions of the curves could be easily changed, and the surface would be updated. If you were using a CAM software that was integrated into Solidworks, it would usually update the tool path as well.