If a humanoid that is identical too a human but stands at roughly 9 feet and has at least twice the strength a human in say the top ten percent of the strength demographic had a bow of equivalent proportion in relation to body size as a medieval English longbow, what would be a realistic draw weight and penetration? (assuming the arrows would become larger in proportion as well as the other parts of the bow, as well as the bow being made of materials capable of using the strength of the pull mentioned above, and that the need for such penetration is fighting other armored humanoids and giant creatures around elephant size)
For real life information, I suggest you watch this show: Agincourt's Dark Secrets Battlefield Detectives. The discussion of the draw weights and penetration of the English longbow is quite illuminating, and quite different from what we generally assume for the performance of the weapon.
Short answer, a longbow delivers @ 38J of energy to the target, failing to penetrate plate armour (see @ 22min into the documentary).
Scaling up bows will deliver more energy, but not huge amounts of energy. Even late Medieval steel crossbows could only deliver @ 200J of energy, while a firearm from a short time later could deliver @ 1000J of energy to the target.
Since the bow is still made of Yew, the OP's bow is likely only going to deliver between 60 to 100J of energy to the target. The larger and heavier arrows will deliver more energy to the target, but may still fail to penetrate plate armour. There could be a knock down effect of being struck by a large, heavy arrow if you are already unbalanced (say from moving around fighting, or balanced on a moving horse), and given enough arrows, there is always the possibility that you will be struck through an opening in your visor or a gap in the armour, but realistically, even a longbow drawn by a giant of a man (or a giant man) isn't going to be the super weapon most people would expect.