I am looking to make an Aasimar Paladin going for Metal Wings. The feat states that you can make a secondary wing attack for 1d4 damage. I was wondering what would happen if you also took the Powerful Wings feat which gives you 2 secondary wing attacks as if you were one size level larger (so a medium creature would do 1d6. Would having Metal Powerful Wings do more damage than the 1d6?
Metallic Wings allows you to make two secondary wing attacks, one with each wing granted by Angel Wings.
Powerful Wings allows you to make two secondary wing attacks, one with each of two wings you already have, usually from your race. The wings granted by Angel Wings certainly qualify. It does not, however, grant wings itself.
Just as you'd need four arms to make four claw attacks, you'd need four wings to make four wing attacks. Since you only have two wings, you can only make two attacks, selecting either Metallic Wings (1d4 slashing) or Powerful Wings (1d6 bludgeoning) for each attack. If you had four wings somehow, then you could make four attacks.
Powerful wings and improved natural attack are both effective size increases. They do not stack.
Metallic wings and improved natural attack would work, but is it really worth spending two feats to get two 1d6 slashing wings when one feat can get you two 1d6 bludgeoning wings or two 1d4 slashing wings? You're basically spending a feat to either change from bludgeoning to slashing or add an average of one point of damage per hit.