The problem with ignore pain was its scaling with attack power and not with damage intake, and while warriors do get rage from damage taken it's not enough in the big picture to say ignore pain scales with damage taken.
Problem with abilities that scale with attack power is usually that tank AP doesn't grow as fast as the damage to the tank across the expansion (as otherwise tank dps would reach crazy levels), and these kind of abilities are "OP" when the damage intake is fairly low but get outscaled fast when the damage intake grows exponentially.
There was similar problem with self-healing tanks when their heals only scaled off AP, so for example they made death strike and frienzied regen scale with damage taken within last x seconds. Maybe some similar treatment should be applied to ignore pain?
Because at this rate if you spend x rage to apply ironfur and it's % damage reduction it's "stronger" the more damage you take, while if you spend x rage to apply ignore pain that has a specific flat cap how much it absorbs it becomes weaker the more damage you take because it depletes in no time.