Yeah, impact is approximately equivalent to Gravity Bow, excepting that impact is not normally available to ranged weapons. However, it is worth noting that impact (or gravity bow) would only raise the die size from 1d8 to 1d10. An effective increase from 4.5 damage to 5.5 on average. At best it's two more points of damage.
I would allow it with the following, but you probably want it sooner than this.
(a) Cast Gravity Bow on Weapon
(b) Cast Wish to permanently imbue it to the weapon
Alternatively, create a new magic item and work with the GM to see if they'd allow, at what cost, etc.
Size Changes, Effective Size Changes, and Damage Dice Progression: I'm confused by how to increase and decrease manufactured and natural weapon damage dice when the weapon's size or effective size changes. There's a bunch of different charts, and I'm not sure which to use.
When the damage dealt by a creature’s weapons or natural attacks changes due to a change in its size (or the size of its weapon), use the following rules to determine the new damage.
• If the size increases by one step, look up the original damage on the chart and increase the damage by two steps. If the initial size is Small or lower (or is treated as Small or lower) or the initial damage is 1d6 or less, instead increase the damage by one step.
• If the size decreases by one step, look up the original damage on the chart and decrease the damage by two steps. If the initial size is Medium or lower (or is treated as Medium or lower) or the initial damage is 1d8 or less, instead decrease the damage by one step.
• If the exact number of original dice is not found on this chart, apply the following before adjusting the damage dice. If the damage is a number of d6, find the next lowest number of d6 on the chart and use that number of d8 as the original damage value (for example, 10d6 would instead be treated as 8d8). If the damage is a number of d8, find the next highest number of d8 on the chart and use that number of d6 as the original damage value (for example, 5d8 would instead be treated as 6d6). Once you have the new damage value, adjust by the number of steps noted above.
• If the die type is not referenced on this chart, apply the following rules before adjusting the damage dice. 2d4 counts as 1d8 on the chart, 3d4 counts as 2d6 on the chart, and so on for higher numbers of d4. 1d12 counts as 2d6 on the chart, and so on for higher numbers of d12.
• Finally, 2d10 increases to 4d8 and decreases to 2d8, regardless of the initial size, and so on for higher numbers of d10.Damage Dice Progression Chart
1
1d2
1d3
1d4
1d6
1d8
1d10
2d6
2d8
3d6
3d8
4d6
4d8
6d6
6d8
8d6
8d8
12d6
12d8
16d6
You're right about the size change. I wasn't thinking clearly. Still, the general point remains. 2d6 is 7 average damage vs 4.5. A maximum damage upgrade of 4. It's was a little too early in the morning for me.
Problem is, spending a round casting the spell to buff your damage is generally worse than just making your attacks unbuffed.
I did the math once, with only 2.5 extra damage per hit, you need a pretty long combat to balance out the damage lost from not full attacking in the first round to cast the spell.
It gets drastically worse as you level up because you have extra attacks from rapid shot and manyshot.
Honestly, gravity bow is only good if it's constant or you can throw up the buff right before combat.