Did some stuff on a training dummy on a hunter with a bit over mediocre gear ( I don't play hunter all that much). 8 4 agi gems on outland items, rest is TW. I'll come back to you about your other points. It's late where I live.
Okay so first things first : gear matters a lot when talking about Hunters. Secondly you got your talents wrong.
I did tests on my hunter (which I mostly play Survival on) and having gear with a bunch of haste which MM does not benefit nearly as much as BM and not having that much mastery (which MM and Surv loves but I couldn't fit that luch in cuz of TBC gear) and BM was inferior to both MM and SV.
Survival gets access to a +13 Agility enchant that far outclasses even a +3 scope on MM/BM and most importantly
https://www.wowhead.com/item=27986/crow-wing-reaper. Now I did have
https://www.wowhead.com/item=39487/dragon-slayers-shortbow with a +3 scope which is about as good as you'll get for bows so the comparison was somewhat fair (does bring a lot of haste and no mastery, to the advantage of BM).
For talents you're pretty spot on for BM though I really don't see why you'd pick
https://www.wowhead.com/spell=393933/war-orders over
https://www.wowhead.com/spell=191384/aspect-of-the-beast.
For MM you need to swap
https://www.wowhead.com/spell=321018/improved-steady-shot for
https://www.wowhead.com/spell=468392/pin-cushion as we aren't starved for Focus and we get much more
https://www.wowhead.com/spell=19434/aimed-shot which makes our damage both smoother and higher. Might seem small but trust me it's light and day.
For Surv what are you doing mate ? Now that's a sabotage if I ever saw one ^^. Here's what a Surv tree should look like :
https://www.wowhead.com/talent-calc/hunter/survival/DAAADVEBAAA
We can't make use of Mangoose Bite reliably at all and having a Wildfire Bomb setup, while interesting, doesn't compare to the almost constant buff that is
https://www.wowhead.com/spell=459964/flankers-advantage with
https://www.wowhead.com/spell=260285/tip-of-the-spear and we don't have enough tools for the spell to be worth the investment at 20.
Now I don't think I know how to use the damage meters as when I stopped my rotation I always stayed in combat for a few seconds, dropping my DPS overall but the numbers I was able to get during the rotation reliably (over 5 min) were :
BM : 500-515 DPS, MM : 555-570 DPS, SV : 610-625 DPS
To be fair I did not expect BM to do as well as it did, being only 10-20% worse than the other specs but as I said my stats were much more in favor of BM so the numbers are a bit skewed in favor of them. I'd expect an higher difference if I had all specs with true BIS.
What skewed your numbers were the talents to be sure and maybe your lack of experience with the class though to be fair Hunters don't have that tough of a rotation. Keep in mind that gear can influence heavily the difference from a spec to another and a spec like Survival has access to tools BM and MM could only dream about.
And yeah as you said Surv is superior in AoE thanks to bomb.
Did priest as well. 7 x 4 int gems. Most of other gear is TW, some is worse. All of this is single target btw. Shadow is good with psychic link at 5~ targets. If you face more than that, swap to holy. BM/MM hunter is subpar to Survival in AOE due to bomb. Sry about low quality images. Had to make smaller because of limits on site.
edit: skipped disc because it's pointless. expect maybe like 60-70% of holy if not less. Smite is nerfed on disc ironically enough.
Now a target dummy isn't a boss. Shadow Priests, while reasonably tanky, are still clothies. Unlike Discipline you will need to spend time healing and using PW : S on cooldown.
While yes, your damage may be superior to Holy in theory, how does that wotk in practice ? Let's admit a well-geared Priest for the following.
Your single-target is way superior and you shouldn't have too much trouble against bosses, at least not that much more than Holy right ?
Any boss/pack/mob that deals over 150 DPS to you (very easy to do) will outdamage even a very powerful shield. Now not only do you need to waste one out of 4-5 GCD to protect, you may have to waste an extra one to heal. Holy not only takes less damage thanks to their better focused will but they also have such a powerful instant heal they can allow themselves to go near death, save
a lot of GCDs and then pop their serenity.
Also as I said with SV, are you sabotaging Holy ^^ ?
In no world is
https://www.wowhead.com/spell=34861/holy-word-sanctify any useful for soloing, let alone any better than picking
https://www.wowhead.com/spell=372307/burning-vehemence.
Try to run the changes to Holy into a dungeon, do the same with Shadow and I'm 100% sure that Holy will outclass Shadow.
Against weaker mobs where you don't need as much healing sure it can work at least as well if not better, I mean you can just DoT up and needing a GCD or two isn't gonna be as impactful if they're spread out but when it comes to overall dungeons, Holy's damage is more reliable which makes up big time for lower baseline DPS.
As a side note, I don't know your stats but if you have any mastery it's wasted on Holy and can skew the damage calc a little bit.
And as another side note, this GCD saving makes Disc deal a lot more damage than you think in dungeons as a well geared Priest can throw 2K shields (4K with crits) that can handle themselves damn well. The damage still isn't stellar but much better than it seems on paper.
As for Rammstein - It recently dropped for me in TW. I think it might've been changed, because it seems to me like the damage drops off the more creatures it hits. Let me know if I'm wrong on that one. I really want that trinket to work.
Sadly yes, the more targets you have, the less it will deal to each of them. With 3-4 targets you deal 1000 damage but they're spread out making it 330 per target. This is true for both the TW one and the regular one.