After seeing several questions and confusion regarding persuasion on the Forum, I decided to do some testing and share the results:The more relevant results are in bold.
From those results, there are 2 possible conclusions:Conclusion 1 - All checks have 2 ways of being passed: either have X persuasion, or have X-1 persuasion and Y of the relevant attribute.
Conclusion 2 - Both the attribute and the Persuasion skill are relevant for the check. But for the purpose of a persuasion check there is a hard cap on the relevant attribute.
Note that conclusion 1 could be false: if the persuasion gives a very large contribution to the relevant stat, large enough that it can't be met with attributes because of the hardcap.
Or, conslusion 2 could be false: and each level of persuasion changes the attribute requirements to pass a check.
This doesn't absolutely answers it, but narrows it down to 2 possibilities.As far as playing is concerned, it appears that attributes over 14 # are irrelevant, checks normally require 14 # attribute + x-1 persuasion, or x persuasion. 14 # is also the largest requirement on equipment so it makes some small ammount of sense, for now.
(edit: stats above 14 appear to be relevant at some checks, probably it increases as the games goes forward, the remaining conclusions remain the same)
For absolute certainty I'd need a check where it is possible to pass at 3 different values of persuasion, that is probably the only way to pinpoint the hardcap and how exactly persuasion contributes to the check.
Until someone datamines the requirements this should settle some confusion for players.