Hmm, just switch from full assault grid (133%) to Phantom axe grid (114%), base dmg change from 45185 to 48769. But after do the math like your example I see my result for new grid is lower than the old one. (105281 vs 104366). The increasing base dmg from Phantom buff is not enough to cover the lost in dmg for 2 slot Assault++.
Well at least the HP is higher and it increases my burst cap so I think it is a good trade. Still hope for Phantom Hammer with ++++ skill, lol.