I know a lot about Rodinol from all three; experiment, theory and reading reports.
I didn't mention contamination or pH effects from water due to the fact that pH and buffer capacity are two different things. A water source at pH 5 and another at 8 if 'pure' water do not have sufficient buffer capacity per se to have significant effect on the other ingredients. Also, a 1% or so impurity in any of the other ingredients is not likely to have much buffer capacity, but may affect pH which is quickly overcome during proper mixing.
The discussion of buffer capacity vs pH has come up enough times in various posts to indicate to me that most non-chemists don't understand the difference.
The use of PAP or PAP.HCl does affect final results for a number of reasons. Most tend to ignore this as well, but it is a not insignificant change. The presence of oxidized products also influences final solution strength to some small degree, but has a bigger effect during mixing of large batches. This is often ignored by non-chemists as well. On the small scale, oxygen entrainment in probably unimportant, but on the 100L or so size batch you can introduce problems. Having mixed developers at both scales, I can tell you that there is a significant difference as a function of batch size.
Read the other thread on Rodinol mixing and formulations.