Results of my weapons testing, with Athens Epilogue before and after an upgrade of attack from 257 to 267:
The acolytes in Steppes cave, which were higher level than I had assumed/wanted, 18, indeed showed a difference, it went from 49 damage to 52. The level 28 ascendent acolytes in the nightfall strike, also showed a difference, going from 147 to 155 damage.
So, at least it's not a divide by ten thing as I had heard postulated, with no effect on lower level enemies. Basically meaning a upgrade from 257 to 267 would have allowed it to have max impact on enemies up to 26.7 level, instead of 25.7 prior. It had an increased damage on level 18 enemies. 3/49=6.1%, 8/147=5.4% on level 28's, so damage actually increased more at the lower levels, although I suspect it is due to rounding, as 2 or 4 damage at the level 18 enemy would have produced a much different percentage.
I wish I had tried some level 4 or some such enemies. Maybe next upgrade. I thought the Acolytes in Steppes were like level 4, but they're level 18 turns out.
I'm sure way more testing has been done elsewhere and/or everybody already knew this, whatever "this" is, but anyway.
The impact rating may have increased with the upgrade. Didn't look like it at a glance, either way AE only has a sliver of impact, but I didn't pay close attention. Doesn't really matter though, the effect is the same.
I guess if all upgrades pay out similarly (more testing sigh) and there are what, 4 damage upgrades per gun? at 5-6% per up, it's 20-24% more damage fully maxed versus not. Sounds reasonable, if a tad disappointing, and it's just conjecture anyway. Having a gun hit ~25% harder after having been maxed isn't too bad anyway I guess.
Rangers, the damage calculation doesn't work that way.
Essentially if you're one level down on the enemy, you receive a 1/3 damage penalty.
Two levels down, it's ~50% damage penalty. Three levels down I think it's 2/3 damage penalty.
This is the main reason for hitting LV30 for the LV30 raid and hitting LV28 for the LV26 raid. (Gatekeeper is LV27, Atheon is LV28)
If you're LV29, you deal 1/3 less damage (and take about 1/3 more damage) throughout the entire LV30 raid.
This is a penalty that almost no weapon upgrade/skill can match.
Maybe you can deal more with a Vex Mythoclast in LV29 over a LV30 character with shitty weapons, but you also have to take into account defense.
You get no benefits for being any level above the enemy, it would just be your weapon's attack rating that's allowing you to hit for more damage.
Granted, lower level enemies have less health than higher level ones so you still get to kill Lv 5s faster than Lv 20s.
The weapon attack rating seem to be linear, but with a catch.
I have several Vision of Confluence from the raid.
So I ran through a test, all guns have same skills.
The 300 damage gun currently crits LV 20+ red mobs for 890, normal hits are 297.
The 267 damage gun currently crits LV 20+ red mobs for 756, normal hits are 252.
The 257 damage gun currently crits LV 20+ red mobs for 715, normal hits are 239.
Using these data points, it appears that the damage is approximately
[(weapon attack) - 80 ] * impact = damage
In the case of Vision of Confluence, this number is approximately 1.35.
[300-80] * 1.35 = 297
[267-80] * 1.35 = 252.45
[257-80] * 1.35 = 238.95
Headshot Multiplier is 3
I also have two Ice breakers that's rated at 260 and 300.
300 hits for 6420 and 1284
260 hits for 5241 and 1049
Headshot multiplier is ~5
For Icebreaker, this impact number is about 5.83
[300-80] * 5.83 = 1282.6
[260-80] * 5.83 = 1049.4
There is probably some minor rounding issues and the 80 number may be a bit off, but it's in the ballpark.
Assumption is LV20+, as the damage formula obviously breaks down for lower levels.
The takeaway is that fully upgraded exotics hit for ~22.2% more than new ones (260=>300, 40 damage increase over 180).
Fully upgraded Legendary weapons hit for ~31% more than new ones (248=>300, 52 damage increase over 168).