Warmog's Armor's regeneration seems overpowering and unnecessary for tanks.
Healing 1% of maximum health per second, which gets huge in high health builds with Spirit Visage, seems to take away a lot of counterplay from tanks.
Since few champions can quickly kill a farmed tank, their deaths in teamfights tend to be either the result of long attrition or the enemy team somehow focusing them without interruption - implying that the tank's team is terrible or he is in a 1v5 situation.
Being able to heal based on percent health is powerful because of this attrition, and it is especially evident in any situation involving chasing a tank.
I do not think that tanks should be able to quickly regain health in the field, barring champion specific cases like Mundo and Singed.
As such, I would reduce the gold cost of the Warmog's to 300 gold (-770) and remove the passive.