Looking at videos for Coil 4, a tank must tank two mobs for approximately 1 minute. They auto attack a Paladin for ~1250 every 2.5 seconds and Rotoswipe for ~1700 every 12 seconds. This subjects you to 4-5 bursts during the timeframe with roughly an 7-8 second recovery phase. Using a 5 second burst window which I consider to be most relevant, Pal is looking at 10900 max and War 13625 (depending on how the mob attack align). A 3k IB is only equivalent to a 2400 heal on a Paladin which is roughly identical to what a Rampart would mitigate on the burst... only it mitigates for an additional 15 seconds and the Paladin maintains his passive defenses while the Warrior does not. You cannot cover these bursts with IB. You can cover one with a large IB and maybe one more with a partially buffed IB if you're lucky. Meanwhile, the Paladin is using superior cooldowns on top of a superior passive to provide far superior tanking throughout the whole time of engagement.