If you don't care about the wall of text backstory, I've bolded the first line of where the part relative to the subject begins, so you can jump directly to it.
I finally obtained the legendary shoulders today (hooray!) after being far, far past due. I already had a 900 CoF begging to be fully utilized, the legendary ring (my first legendary, thankfully), and one Meta cooldown-reduction relic; as it stands, my Meta CD is now 1m50s to 2m05s (on average) depending on Fury generation. I could probably ditch the relic and still be okay, but I'm keeping it in case I eventually get the belt and can swap the ring; I have both a heroic Meta CD relic and a heroic WF Critical Chaos relic in my bags in case I decide to change my setup in either direction. Anyways, I'm getting off track and need to keep my ADD at bay.
I had to give up my legendary neck in order to use the shoulders, which actually hurt quite a bit; while the legendary "special" ability on the neck contributes nothing to our DPS (outside of keeping us alive; dead doesn't deeps**), it's still incredibly powerful. On top of that, the stats are just insane; most of my legendaries are roughly a 40% upgrade over their non-legendary alternative, while the neck is a stunning 70% upgrade (compared to my two 885 and one 890 alternatives; not the best, but certainly not terrible). Because of this, my secondary stats changed quite a bit when I swapped from neck to shoulders. I had some work cut out for me to balance things out.
I had to play with various equipment sets in order to optimize my gear after swapping to shoulders; I had been using the tier shoulders, so this meant swapping around my tier as well. After quite a few different combinations and several simcraft runs, I came to one conclusion; our 4 piece bonus really isn't all that great as you begin to acquire better, higher item level gear that has crit.
Off the bat, the four-piece is great - several NH pieces are lacking crit yet are still upgrades, leading to many of us losing a substantial amount of crit while gearing up. Gaining 10% on Chaos Strike was practically a requirement to maintain even decent Fury generation in many cases.
However, once you begin to obtain higher item level pieces that have crit, whether it be from Nighthold or lucky procs from M+/Emerald Nightmare, you'll slowly regain that crit you'd previously lost while still maintaining a high item level and appropriate Mastery/Haste.
What it boiled down to was this; I could equip the four-piece with one set, or equip the two-piece with another. If I went for the two-piece, I gained about 6-7% baseline crit and 3-5% Mastery depending on pieces. Haste varied, but I could gain anywhere from nothing to about 2%.
So, what's better in the end? 10% higher critical strike chance on Chaos Strike, or x% higher crit chance on EVERYTHING? If you look at crits-per-ability, a critical Chaos Strike is much more beneficial than a critical Blade Dance or Felblade; the 50% fury refund is a tremendous part of our overall Fury generation. However, there is still an amount of damage done by abilities outside of Chaos Strike that will outweigh this. I can't give an exact figure as I haven't done the math, but one could assume that there is an appropriate value to insert into "1% Chaos Strike crit is equal to x% overall crit", or "1% Chaos Strike crit is equal to x overall DPS" -- it's just a matter of finding what "x" equals for you.
You will eventually get to a point where passive crit is better than the 10% bonus, and this amount will obviously be less than 10%, as 10% passive crit is equal to the bonus anyways. For me, 7% seems to be around that magic number. You've also got to consider the gain of Mastery (and even haste), further contributing to this breakpoint. Now, if swapping out those tier pieces leads to a statsheet crit gain but doesn't increase your Mastery, let alone decreases it, you're obviously going to be looking at entirely different values. That aside; if 7% overall crit contributes more overall damage than 10% crit to Chaos Strike, then gaining only 5% crit but also 4% Mastery could also be a breakpoint where it's contributing more damage than the CS 10%. It's all in the math, which I or somebody else will have to do.
So, while many of us gear up around that 4 piece bonus and don't look at it as really even being an option, consider that there is a mathematical breakpoint where your 4 piece becomes less beneficial than 2 piece + offset pieces. As you progress throughout NH, don't be afraid to roll for items that conflict with your tier bonus. If you wear the tier gloves and a great pair of gloves drop, roll for them -- as long as you communicate to your loot master why you're rolling, and that a raider that can use the item immediately should get dibs.
Another thing to consider is the number of tier pieces we have without crit; legs and gloves are a good example. These are prime pieces to replace when ditching your 4 piece, as you lose no additional crit rating. If you ditch the chest, you're not only losing the 10% crit bonus but the crit contributed by the chestpiece. Even if your replacement chest has equal crit rating, it's still a greater loss than replacing a piece of tier without it if you look at it as "total crit contributed by my equipped tier". When doing this, you want your two-piece to consist of pieces with additional crit rating whenever possible; us having pieces without crit plays a large part towards how this is possible without needing to outgear current content to a tremendous extent.
tl;dr version: Wearing two pieces of tier, both with crit, can sometimes be a DPS gain over automatically going for four-piece regardless of your alternative pieces for those tier slots. If you have to choose between "4 piece, 38% crit, 32% mastery, and 9% haste" or "2 piece, 45% crit, 34.3% mastery, and 9.8% haste", you could very well be better off sacrificing your four-piece bonus.
Do some math yourself. Run some sims. Don't blindly assume that wearing four-piece will absolutely be better for you, and just the same, don't automatically assume you can afford to ditch it right away. Because it's a flat stat gain (and also because it's only towards one ability), its value will diminish over time as you gear up. For most of you, I'm sure wearing four-piece is still beneficial. As I said before, many of us lose overall crit when we begin to gear in the Nighthold. For the rest; take a look at those alternative pieces for tier slots that have been collecting dust in your bags. There's a chance you could be doing more damage while wearing less tier with pieces you already have.
Over the next few days I'm going to try to do or find some of the math needed to determine precisely what amount of stats are worth trading your four-piece bonus for. If any of you have any questions on whether it's worthwhile for you, or if any of you would simply like to contribute, we can do it together in this thread. I have another thread where I help people with WeakAuras that I'm far too behind on (my internet provider decided to perform maintenance tonight right as I was starting, I'm currently tethered to my phone for internet) and I'd like to knock some of that out first, but the second I noticed I gained a small amount of DPS by dropping down to two-piece I immediately became interested in sorting out the math behind it. As I know more, you will!
Sorry for any typos or grammatical errors; it's 5am and I've been up since 8am yesterday >.< Good night! :3
**I totally just came up with "dead doesn't deeps" off the top of my head and while I cringe at the use of "deeps" over "dps", I'm relatively proud of it and am claiming total ownership of said quote. Use without permission is strictly prohibited. Royalties incoming. I'm going to be TOTALLY loaded.