Prestige Class Ability: Mighty Weapon

[Balord]

I'm designing a prestige class for my campaign world called "Iletheral Guard" that is similar to a Devoted Defender who specializes in a particular weapon. Instead of giving them Weapon Specialization with their weapon of focus, I wanted to give them something more unique called Mighty Weapon.
Mighty Weapon - you have been trained to deal more damage than normal with your "weapon of choice". Refer to the following table to determine what dice to use when rolling for damage.
Normal ---> Mighty Weapon
d6 --------> 2d4
d8 --------> d4 + d6
2d4 or d10-> 2d6
d12 or 2d6 -> 2d8
I'm thinking of giving this as a special ability at level 5 of my prestige class. Does this seem a fair ability, or overpowered?

 

[CRGreathouse]

Instead of giving them Weapon Specialization with their weapon of focus, I wanted to give them something more unique called Mighty Weapon.

First, "more unique" rubs me the wrong way.

Second, this ability is fine if it's what you want it to do -- that is, it will increase the power of larger weapons more than smaller weapons. Increasing die damage is better for, say, greatswords than daggers.

Do you want the class to encourage larger weapons?

 

[Khaalis]

Why not simply use the rule that the weapon deals damage as if it were one size larger?