Kefka Sucks. Kuja Is Better

[Forsaken Lover]

He was well aware of his own limits given the fact he spent the first half of the game scurrying around hoping Garland didn't notice him. Then when Garland did catch on to his plan he becomes obviously distraught and desperate. He even went so far as to emphatically declare his own power was nothing to Garland's.

While being ignorant of the fact he will eventually die can be somewhat chalked up to Kuja's narcissism, he's not completely detached from reality. He recognizes he has limits. I think he just believed that more power = breaking those limits. As I noted earlier his search for power was a way to be who he wanted to be. If he had understood the fact that no amount of power could realize that ideal there'd be no game as he'd have no motivation.

That is my interpretation anyway. There's nothing to say definitely one way or the other.