Holy stuns, a less geared tank is even more reason to holy since stunned enemies aren't attacking. The tank HP is a non-factor and people need to stop repeating the nonsense that 2000 less hp stops a healer from DPSing. Holy is not only DPS its a way to stop the tank taking so much damage.

And especially if it's a WAR since that DPS with the right cooldowns+overpower spam practically allows them to self-heal. If you are struggling to heal a tank they are not using the right cooldowns regardless of HP. 2000 more HP buys you 1-2 seconds until they drop, if damage is outpacing healing there is literally nothing you can do. If you can DPS its because the tank knows how to be effective with cooldowns, if you can't it's because they are terrible, nothing to do with how much HP they have.