Mythic Power Attack says...
When you use Power Attack, you gain a +3 bonus on melee damage rolls instead of +2. When your base attack bonus reaches +4 and every 4 points thereafter, the amount of bonus damage increases by +3 instead of +2.
In addition, the bonus damage from this feat is doubled on a critical hit, before it’s multiplied by the weapon’s critical multiplier.
You can expend one use of mythic power when you activate Power Attack to ignore the penalties on melee attack rolls and combat maneuver checks for 1 minute.
It doesn't mention that two handed weapon (“THW”) damage is increased but the normal Power Attack rules should still apply (i.e. +50% for THW) since the feat doesn't say otherwise.
So wielding THW's should equal +4 damage per -1 to the attack, advancing as normal for each 4 levels gained.
Is that correct?