My math for it was +0.83 dmg per attack, but now I want to re-check.
1d6 average, instead of being (1+2+3+4+5+6)/6, becomes (3.5+2+3+4+5+6)/6 (the 1 is rerolled for a d6, which has an average of 3.5). So 3.916666. Do - 3.5, x2 for the 2d6 of a greatsword and... yeah, not good.
It's...