Specifically grouped content.
When the skill cap is low, one logical conclusion once someone has reached their individual potential is that they would spend their time doing other fun things. However, if the goal is to complete group content, the difficulty of assessing that the skill cap has been reached manifests as effort to contribute to the group's success by progress in areas that do not require skill.
In this sense, offering non-skill increases in effectiveness, either permanent or temporary, are only harmful to enjoyment if goals are set which cannot be accomplished solely by increase of skill. From a competitive standpoint, if every individual player eventually reaches a personal skill cap, then any interaction has the potential to demonstrate incremental improvement without the need for a long-term goal.
By increasing the perceived skill cap and significance of each isolated victory, there can be greater 'depth' of non-skill progress offered (either permanent, or temporary) without the feeling of those elements being required to participate in group content.
This, in turn, requires not only the possibility of discerning whether an increase in skill has taken place, but also that the individual can benefit from this progression, which is not measured by the game. The easiest way to do this is, as mentioned before, congruence of individual goals with group benefit.
The second implication of the skill cap is social patterns of voluntary grouping. A higher skill cap, meaning greater variance in performance, implies easier completion by a group that's better than normal. However, there are two possibilities that may prevent extreme divergence of group performance in the population that would complicate content difficulty tuning: a higher skill cap means less reason to avoid the situation of the previously-mentioned non-skill progression elements feeling required in a group that is not progressing, since instead of "farming more potions" someone can just play better at the moment an encounter takes place, if the reason for playing a game is for challenge instead of in-game rewards; and also when rest of the game has sufficient justification to improve even for those who do not have challenge as the primary goal, decreasing the variation in performance in the population by increasing average competence as time goes on.