Leadership consists of Inspiration and Determination.
On Supernova the companions can die, so to make them more resilient I have set Determination to 100 (When companions kill an enemy, they gain 50% health). Inspiration set to 60 (Companion skill bonus to player skills +100%) to efficiently use skills for e.g. dialogues or stealth without increasing the skill points of your own. There are no diminishing returns but it will take some levels to get Determination to 100 skill points (which you could use for other skills). It is a choice how you want to play the game from the beginning of course.