Warrior main tanks in non-scrub guilds have up to 24k HP right now with Sunwell gear.
T6 gear probably won't be replaced until 80, like McClain said. I was still wearing T3 pieces when I hit 70 and didn't start replacing stuff until I was getting D3/T4 gear. Some of the old "holes" like neck, rings and trinkets got replaced early on in the 60s, so I'm expecting the same thing to happen.
My main concern is keeping hit capped.