That ultimate cooldown reduction rune seems pretty legit on Kassadin

Kass ult cd at level 16 is 2 seconds. If he has 40% cdr it goes to 1.2seconds. Ultimate hat can get you to a max of 52% cdr if i remember correctly due to it stacking multiplied not additiv. This would bring him to a cd of 0,96 seconds. 1,2->0,96 is not that much of a difference.

Don't forget the unlimited mana for 5 seconds on takedown! (it's in precision)