Hi Pastuzo. There's no harm in asking, but if there's one myth I want to bust, it's this one.
Getting better gear unquestionably increases your effectiveness as a tank, against previous, current, and future tiers of content.
Gear affects us in 3 general ways - higher health total, better mitigation, and increased attack power. Let's touch on each:
Higher Health
Higher health is complicated. It's generally good for us, but it's offset by reduced RFDT (rage from damage taken) rate. There is a window of incoming damage and healing values where a player with lower health will live where a player with higher health won't, but generally higher health makes us more resistant to spike damage and gives healers more time to heal us. There's been a lot of discussion of health in pages 74-77, so I won't do a deep dive, but here's a couple posts if you want to read more.
http://www.mmo-champion.com/threads/...1#post40523301
http://www.mmo-champion.com/threads/...1#post40540728
Mitigation
Mitigation is anything that passively reduces the average percent amount of damage we take per second - armor, versatility, dodge, parry, block chance from mastery.
First, it is better to not take damage than to take damage and get some rage for it. You need to take 80% of your health to gain enough rage to generate an IP that will generally absorb damage equal to 20% of your health. That's not a good trade. Assuming there are no bosses that regularly hit us for 100% of our health - and given the presence of tanks that recover with healing, I think that's a safe bet - there's no situation where keeping health isn't better, so anything that reduces DTPS is an improvement.
Second, mitigation multiplies the value of your IP absorbs. Say Tank A and Tank B are identical, except Tank A has enough armor to take 75% as much damage per attack as Tank B. If they both put up a 100,000 IP absorb, a boss will need to deal 100,000 to Tank B to consume his absorb, but 100,000 / 0.75 = 133,333 to Tank A to consume his absorb.
So mitigation is double plus good.
Attack Power
Any increases to attack power (from strength or mastery) increase the size of your IPs. Your versatility value also increases your healing done, which includes IP. Increases to those stats make you stronger without any downside. I'm going to include haste in here as well - its benefit is harder to measure, but it's all upside.
Dealing Damage and Holding Threat
Bonus section! Back in the bad-old-days of WotLK and BC, tanks would take gear off in low level content because we got rage from taking damage and had to pay rage to deal damage. If we weren't taking damage, we couldn't deal enough damage to hold threat. That is no longer the case - rage is only spent on active mitigation (and Focused Rage, which isn't a big deal). Therefore, there is nothing about getting better gear that reduces your ability to deal damage or hold threat in any way. You could be taking 0 damage and hold threat fine.
As you can see, all the benefits from better gear are good. Health is the only weird one, and the downside to higher health is outweighed by the benefits to mitigation and AP that come with it on gear.
Therefore, better gear is always an upgrade. QED.