Well, for whatever it's worth, I'll tell you what I remember about this. I remember a lot of confusion about this, and that was supposedly cleared up with the errata.
The confusion was over how to handle the "1.5x" damage bonus. The solution was the errata.
p. 8, Power Lunge
Insert to the end of the 1st sentence in Benefit:
regardless of whether you’re using one- or two-handed weapons.
This makes the first sentence read:
A successful attack roll during a charge allows you to inflict double your normal Strength modifier in addition to the attack's damage regardless of whether you’re using one- or two-handed weapons.
Let's repeat that, with a little highlighting:
A successful attack roll during a charge allows you to inflict double your normal Strength modifier in addition to the attack's damage regardless of whether you’re using one- or two-handed weapons.
It looks like 2x str damage to me.
It's especially clear when you consider that the errata was written solely to clear up the issue of what to do when your normally got 1x or 1.5x str damage. In that context there seems to be little doubt that this 2x str damage replaces the 1.5x str damage from using a two-handed weapon (or one-handed weapon in two hands, of course).
I know the language could have been clearer, but the errata folks were obviousy trying to make the smallest change possible to clear up the two possible ways to interpret this feat.
I see no way of reconciling the
additional language with an interpretation of 2.5xdamage from two-handed weapons. If you went that route, wouldn't the additional language be meaningless?
Unless I'm really missing something, this rule looks pretty darn clear.