It seems like the higher gear levels are focusing more on secondary stats (armor penetration, etc) and less on stamina and agility. This has led to me sticking more and more PvP gear in my tanking set.
I'm wondering what the effect of filling more and more slots with PvP gear will be? That resilience must mean wasted item-budget somewhere. Will I eventually end up with threat issues? Mitigation issues? Or is PvP gear fine for raid tanking?
This site has a long habit of alternately hating and envying PvP gear for bear tanking. This season is no different, though there are at least a lot of options out there that are competitive with PvP (until you get to weapons, that is).
And it's a very good observation that the gearing has more of an emphasis on secondary stats (particularly armor pen) and less on base stats, especially compared to PvP gear. This is a really easy (and sad) thing to explain. As the item level goes up, not only are more stats on the piece, the items tend to be better itemized. This is by design; they want people to want the hard mode gear over the normal, they want people to look and drool, and they want in general there to be significant upgrades. The easy way to do this most of the time is to give somewhat poor itemization early on, then improve it so that by the time the endgame hits, people are fairly happy with the gear.
So why is this a problem for bears? Well, early on you get things like Hood of the Exodus - which has poor secondary stats and an abundance of stamina, something no rogue or cat wants. As gear advances, stamina is reduced in priority on this gear and armor pen is increased - exactly what rogues and ferals want. The problem is that bears want badly-itemized leather. They want a ton of expertise (no rogue does), they want a ton of stamina (most rogues don't), they want some hit (rogues like more), and they don't really care that much about AP, arpen, crit or haste (rogues care much more about this stuff). Thus as gear gets better, it tends to get relatively worse for bears.
This is why Vendorstrike is good as well; who in their right mind would want a weapon that has 180 stamina and only 90ish agility? It's horribly itemized for feral cats and hunters and most DKs and ret paladins, but it's great for the bear.
By comparison, PvP gear emphasizes all of these points for bears. It wants high stamina, it wants high survivability, and it doesn't care at all about secondary stats. It does waste some item points on resilience, but is it a big deal? Let's see by comparing the Relentless set with the T9.245 set. Now, the Pvp gear is slightly higher in itemization level, so that will make a difference, but it won't be ridiculous. I'm also assuming going stamina normally and only going after slot bonuses when they're easy and good and would make me not go out of my way (6 agi with one red gem will cut it; 4 agi will not, and no yellows will period).
Relentless Gloves vs Runetotem Gloves:
+9 armor, + 6 agi, 21 stam, -27 AP, -59 hit, -4 crit
Relentless Head vs Runetotem Head:
11 armor, 8 agi, 28 stam, -4 AP, -74 expertise, -10 crit
Relentless Legs vs Runetotem Legs:
12 armor, 8 agi, 13 stam, -4 AP, -74 hit, -10 crit
Relentless Chest vs Runetotem Chest:
13 armor , 8 agi, 28 stam, -36 AP, -4 crit, -90 haste
Relentless Shoulders vs Runetotem Shoulders:
10 armor, 6 agi, 21 stam, -27 AP, 63 crit, -59 arpen, -67 haste
Total difference between relentless and Runetotem:
55 armor, 36 agi, 111 stam, -98 AP, -133 hit, -74 expertise, 35 crit, -59 arpen, -157 haste
So the answer is yes - you are spending a lot of budget on resilience, and you are losing a fair amount of threat stats. The AP loss is negligible but the loss of 133 hit, 74 expertise and 157 haste is fairly large, even with the extra agility and crit to make up for it some. Don't overlook the armor either; 55 armor pre-buff corresponds to almost 400 armor with bear buffs. Resilience is useless outside of Thorim and other gimmicky fights like that.
Is the loss of that much threat stat worth it? Could be depending. Note that this to me speaks to cherry-picking the best PvP gear and going with tier gear otherwise. The head, for instance, is a great spot to pick up expertise on the tier gear - and the legs lose the least amount of stam on the tier gear and give hit. The chest by comparison only grants haste, and the shoulders haste and armor pen.
To even further muddy the mix, note that getting things like the chest and gloves allows for a kitty-specific gemming strategy in those slots that may allow for interesting mixing and matching; for instance, I have the PvP gloves, and I gem/enchant them for stam. I have the heroic Sunreaver Assassin Gloves and they're gemmed for agi and enchanted for agi. Depending on the fight, I can use either; I use the Sunreaver for Anub'arak and the Relentless for Beasts.
Also note that you may be fine without all that threat anyway. It very much depends on what your gear is and more importantly, what your raid is like. Heck, it depends a lot on your talents too. Whether it's the right choice for you is really going to be your decision. Just bear in mind that the pvp gear is certainly competitive, and from a pure mitigation/health perspective is definitely superior.