Siegfried's Morale boost is not a direct upgrade of Gilgamesh's equivalent skill. Siegfried's skill adds a very small amount of rage reduction to every attack, while Gilgamesh's skill directly reduces the rage meter by 20%. Gilgamesh's skill is much stronger in practice. The skill I was referring to, and I assumed you were referring to, was Siegfried's damage skill that does high damage to Raging enemies.