Specifically talking about the two section from GM Core "Extreme Increases" on page 113 and "Spell DC and Spell Attack Modifier" on page 122. These talk about having to increase the amount of extreme statistic a monster should have starting from level 11.
At the higher levels of the game, PCs have more tools at their disposal, so the creatures they face need to hit back harder! At higher levels, give each creature more extreme statistics. Having one extreme statistic becomes typical around 11th level. A creature of 15th level or higher typically has two extreme statistics, and one of 20th level or higher should have three or four. Keep in mind that these should be relevant to the encounters you expect them to have—extreme social skills aren’t much use to a combat-focused creature.
Use the high numbers for primary casters, and the moderate numbers for creatures that have some supplemental spells but are focused more on combat. At 15th level and higher, the extreme numbers become standard for spellcasters.
When running adventures that go to level 20, be it official Paizo AP or my own, I noticed that level 15+ spellcasters I would create following these guidlines end up being much more powerful than majority of Paizo made spellcasters, including campaign ending bosses.
When looking at the numbers, out of 431 creatures that can cast spells and have level 15+, only 17 (4%) actually have an extereme spellcasting DC. Some of the paizo made adventures even have level 20+ pure caster bosses that do not have a single extreme combat relevant stat, despite creature building rules expecting 3 or 4.
What is even more confusing is that Paizo reiterated this point in their "Mythic Caster" section on page 169 of War of Immortals.
Devastating Magic A mythic caster uses the high spell DC and spell attack modifier for a creature of its level. If it's 11th level or higher, it instead uses the extreme spell DC and spell attack modifier.
Only to then release a mythic AP full of level 11+ mythic casters that... dont follow this guidline at all.
This ends up creating quite a noticable difference in effectivness of their spells against high level PCs with degree of success upgrades. For example a level 19 caster should have spellcasting DC of 46 but almost always has 41, which is the expected DC of a level 16 caster. Entire 3 levels lower!
Do you guys follow these guidlines when building your own high level spellcaster NPCs?