What I didn't realize at first - mainly because the skill description does not really explain it - is that Entangling Shot actually damages 2 enemies per shot (or 4 with the Chain Gang rune). So that's 75% * 2, which suddenly does not seem that bad, especially with the slowing effect. That makes it viable at low levels as a "mini-AoE" weapon, similar to Hungering Arrow. In later levels the slow effect becomes the more important thing, and can really make a difference.
I personally think every Demon Hunter needs some way to slow down enemies. If you're not fond of other slowing skills, this might just be the one for you, though I agree that in the long run this causes less single-target damage than any other DH skill, and less AoE damage than other DH AoE skills.
Frankly, I don't rely on my hatred generator for damage, only for - well - hatred generation :) having it slow multiple enemies at once is a definite plus.
Companion damage scales based on attack speed. This was confirmed by Bashiok and tested by theorycrafters, who arrived at these numbers*:
APS Companion Damage
1.00 30%
1.34 40%
1.40 42%
1.43 43%
1.60 48%
1.84 55%
*Calculated by looking at the in-game tool-tip, which changes based on the APS of the character.
Note: The wolf, being a shorter duration (25 seconds), does twice the %.
That is, the companion does the % of your weapon damage based on your APS. If it follows this logic of being a channeled DoT (which is what most theorycrafters have come to agree), then based on Bashiok's post, the tick rate (how often it attacks) increases as your APS increases. As for speed it attacks, it would behave like any other DoT in this situation. It would attack once per cycle for its inherent percentage, so if your APS is 1, it attacks once per second for 30% weapon damage. If your APS is 1.6, your cycle time decreases to 0.625 seconds per attack (1/1.6), so it would attack once every 0.625 seconds for 48% weapon damage. If it follows this model, your weapon damage would have nothing to do with companion damage, but its speed (and therefore indirectly, its dps) would affect companion damage.
As for the boar companion, it says on the wiki that it "strikes multiple enemies with every swing."
Edit: After some testing with a level 60 demon hunter, it appears that the percentages are roughly correct. For the ferrets, even though they are animated to attack separately, it seems the total damage is split between the two. The boar is really hard to judge, but as far as I can tell, the damage is the same as the others, just as a cleave attack that hits enemies in front of the boar for full damage to each.
All this information is gathered by personal experience or by experiences posted on Diablo forums that were confirmed by more than one person. It's difficult to get exact answers until everyone starts posting their findings, since Blizzard rarely explains how game mechanics work.
Best Answer
I tested it and it is most likely that it is the 185% so it just adds 70% (115+70)
to the data:
normal hits without pierce made up 50 dmg that are 115%. so 100% wpn dmg was about 43.47 i'll take 44. 70% from 44 are 30.8. so one pierce would make 80 dmg, 2 would make about 110 dmg and 3 would make about 140 dmg
I shurely collected more data. one Pierce made about 80 dmg 2 pierces made about 110-120 dmg and 3 pierces made about 140-155 dmg
As you see it is the 185% weapon dmg.
All data was made with a 16.5 dps 2 handed crossbow 14-16 dmg without any other gear on a mallet lord in act4 hell with my DH lvl 60. Additionaly i had no dmg buff passives active (had hot pursuit, brooding and tactical advantage)