@Baskakov_Dmitriy That's actually a good point. 4x Greatsword attacks at +9 (2d6+7) yield 33.6 DPR before increases. AND they're 17.5 DPH (per hit), so will go better through DR.
@Draco-S Since there is no case for TWF while actually holding a weapon with two hands, I'd assume that one can normally wield a GS with two off-hands for 1.5x STR damage. So you don't need Double Slice.
Also, saves some money on weapon enhancements. Someday you'll want +1 on your weapons.