A custom pipe, using precise engine build parameters along with some testing should be better than any off the shelf stamped pipe. If is is not, it needs to be re-evaluated and maybe some of the parameters were not taken into consideration. A custom pipe should have a better power delivery in the intended rpm range as it provides a stronger return sound wave (and pressure) back into the combustion chamber before the exhaust port closes on the way to TDC. Some of the flaws in how an engine makes power I believe can be fixed with a correct pipe. A pipe is not really a one size fits all. It can work well within a range of engine parameters (ignition timing, compression, port timings, blow down, intended rpm range, rpm drop between shifts, estimated operating temperature, etc, etc) but not be optimum for any particular engine.
I don't think any pipe builder would dispute this. In the real world when testing there may be some unknown, unforseen variables to deal with, and that is where the cut, weld, and retry method may come into play.