Take a look at the top tanks in the world doing Heroic content. I bet you won't find 1 pure stamina gem (Only in extreme conditions). Most tanks in heroic guilds have very low HP, but around 50% block (for warriors) and high avoidance.
You don't need high HP, you aren't a sponge, or you aren't meant to be. High hits aren't THAT high anymore and on-top of that they are mostly predictable so you can use your cooldowns on them.
I would argue that you need more HP in heroics than Raids. Raids you have a lot more buffs and control than 5 mans. 5 mans you generally take a lot of damage from some trash.