The data for that paper is interesting, it is outer flesh, not whole plant in many of the cases, there have been no published reports of san pedro cacti having a mescaline concentration of 5% in the whole plant, just outer skin. However those intimately familiar with these plants tend to think that 6% whole plant is likely the upper range of potency...
The pachanoi from matucana was provided for the study by someone who grows cactus for a witches market, they also provided a fat blue peruvianus but this was not included in the study for specific reasons.
The highest concentrations of mescaline in San Pedro cacti come from pachanoi and bridgesii. Peruvianoids tend to be inferior plants in terms of data and anecdote. Of course it should be noted that the standard pachanoi in circulation is rather weak and does not seem to even be typical of pachanoi at all.
Erowid, something I am not a fan of, has a rule of thumb for dosage, it says something like 18 inches of cactus as thick as a wrist for dosage, if you did that with a potent clone you would regret it. A good clone works well with a dose about the size of a medium sized apple, 250 grams should be plenty for these clones fresh, that is a little over half a pound. If you followed the Erowid criteria with a potent clone you could easily ingest over 2 grams of mescaline, few people would repeat that a second time.
If you take the Reti paper into consideration you have to note that outer skin may only contain a third of the alkaloid in the plant, but at a much higher concentration overall than the whole plant.
KK is an old alcoholic, he also has a huge peyote garden and a huge San Pedro garden, his blunders and issues do not seem to be on purpose, likely the KK242 issue comes from a simple mix up of a single batch of cuzcoensis seed. Clones obtains from him with the label kk242 tend to be quite nice actually, but not cuzcoensis like at all. But then not everything bearing the name cuzcoensis is weak or inactive, just most of it.
As for hybrids... anecdotes about extraction indicate decent recovery of mescaline, anecdotes about potency indicate an activity somewhat in between the parents, so an offspring of two strong parents should be strong.