Since I'm in the mood for some math, I would like to answer your question. Should OP or anyone else find any mistakes in this post, they can always correct me.
Let's see how much damage arp actually gives you. From this thread:
http://forum.warmane.com/showthread.php?t=298787/
we can say the following:
Boss armour = 10643
Boss armour (reduced by Faerie Fire + Sunder Armour) = 8088.68; Arp Cap = 7773.73
1400 arp = 100%
1120 arp (in your case) = 80%
-> Effective Armour (with 100% arp) = 8088.68 - 7773.73 = 314.95
Effective Armour (with 80% arp) = 8088.68 - (0.8*7773.73) = 1869.696
-> DR% (with 100% arp) = 314.95:(314.95+15232.5) = 0.02 => DA% (with 100% arp) ≈ 98%
DR% (with 80% arp) = 1869.696:(1869.696+15232.5) = 0.109 => DA% (with 80% arp) = 89%
In other words, if you use that trinket, your Mangles, Shreds, Ferocious Bites, and Autoattacks will do 9% more damage for 10 seconds, and then the trinket proc will be on a 45 sec cooldown. Bleeds don't get affected by armour. So, I dare to say that it's not worth it. Arp after 1400 is a waste, and it has always been that. There's a trinket, that similarly to
http://wotlk.cavernoftime.com/item=50198 gives crit, and that is
http://wotlk.cavernoftime.com/item=50342. However, the latter trinket is more valuable because att power(10% increase from Heart of the wild) enhances not only the aforementioned attacks, but also our bleeds - and for 15 seconds! Not to mention, the crit bonus is higher, which increases the odds of you landing crits with your combo point generating abilities.
But let's forget about crit for a moment. Let's concentrate on the pure damage. For this case, I'll use my attack power from my Icecrown toon, which is 10722 unbuffed. I'll use this number in both cases - with 100% arp and Needle trinket and with 80% + Skull.
100% arp
Shred = (AA*2.25 + 666)*Naturalist*Mangle*SR*Rend and Tear
According to some sources, the cat’s AA damage is calculated by multiplying our AP by 1/14 and adding 55 (average pawn damage).
Shred (non-crit) = 5736; if I have to take DA% into consideration -> 5736*0.98=5621.28
Shred (crit) = 12619 (with Predatory Instincts) -> with DA% 12366.22
FB 5CPs (Rend and Tear increases the crit chance by 24, so I’ll might as well concentrate on the critical strike):
FB = (120+290*5+0.35*AP)*Feral Aggression*SR*Naturalist*2*Predatory Instincts
FB = 19701 -> with DA% 19307
80% arp
Attack Power = 10722+1210 (AP from Skull trinket) = 11932
Autoattack Damage = 907
Shred = 6179 -> DA% 5499.31
Shred (crit) = 13593.86 -> DA% 12098
FB = 21268.9 -> DA% 18929
The numbers don't look in our favour, but we need to consider other factors:
1) Number of buffed shreds in the first case vs the second in the span of 15 seconds. In order to have a clearer picture, we have to assume that we will inflict the same amount of shreds within that time frame and compare the damage. It's important to note, that after Needle's proc ends, your arp is back to 80% (DA% = 89%). But, how many Shreds are you going to use during Needle's 10 sec CD and 5 seconds out of the buff? After all, you can't spend all of your energy on that, you'll need to refresh Rake and quite possibly Rip as well? Let's say 2-3 buffed Shreds and one unbuffed, i.e. with the regular 80% arp?
2*12366.22=24732.44
3*12366.22 = 37098.66
Shred (unbuffed) = 11230.9
VVhat about during Skull's 15 seconds? Honestly, I have difficulties answering this question, because here you might refresh Rake twice, and you could need energy for refreshing Rip or Savage Roar. However, for the sake of the example, I will keep the number of Shreds the same.
3*12098 = 36294
4*12098 = 48392
Let's compare them:
2*Shreds (buffed from the Needle trinket) + unbuffed Shred = 35963.34 < 3*Shreds (buffed with the Skull)
3*Shreds (buffed from the Needle trinket) + unbuffed Shred = 48329.56 < 4*Shreds (buffed with the Skull)
2) I've mentioned bleeds being enhanced by the Skull, so let's include them here:
Rake = (AP/100+176+3*358+AP*0.18)*Savage Fury*Mangle*SR*Naturalist
Rake unbuffed = (107.22 + 176 + 1074 + 1929.96)*Savage Fury*Mangle*SR*Naturalist = 7502 or up to 16504.4 (if everything is critical)
Rake buffed = (119.32 + 176 + 1074 + 2147.76)*Savage Fury*Mangle*SR*Naturalist = 8026.96 or up to 17659.3 (if everything is critical)
Rip 5CPs:
Rip = (36+93*5+0.05*AP)*11*SR*Mangle*Naturalist
Rip (unbuffed) = (36+ 465 + 536.1)*11*SR*Mangle*Naturalist = 21697 or up to 47733.54 (if all 11 ticks are critical)
Rip (buffed) = (36+ 465 + 596.6)*11*SR*Mangle*Naturalist = 22962.77 or up to 50518.1 (if all 11 ticks are critical)
I'm willing to accept, that you don't refresh Rip during the 15-second buff from the Skull, but you will refresh Rake, which will grant you 524.96 up to 1154.9 damage (if all ticks end up being critical).
3) So far, so good, but what about FB? If you compare the FB (Needle) vs FB (Skull), the latter will do 379 damage less. Thankfully, this is compensated by the bonus Rake damage.
4) I'll ignore the damage from Mangle, because we use it once per minute, but here I will take the Autoattack damage into consideration.
AA (unbuffed, no armour damage reduction) = 820.5*Naturalist*SR = 1200.39 or 2640.86 (crit)
AA (buffed) = 907*Naturalist*SR = 1326. 94 or 2919.27 (crit)
AA (no AP buff, but 100% arp) = 1176.38 or 2588 (crit)
AA (AP buff, but 80% arp) = 1180.97 or 2598.1 (crit)
So, there's a small buff here as well. As you can see, the Needle trinket is unarguably a weaker gear choice that VVFS.
[edit] Here are some sites, where I found information about some formulas:
https://wotlk.evowow.com/spell=48574#see-also
https://wotlk.evowow.com/?spell=49800
https://wotlk.evowow.com/?spell=48577#see-also
https://wotlk.evowow.com/spell=48572?forums#see-also
https://www.wowhead.com/forums/topic/142010
