At 8k hp with Gryphonskin, you're in full i90 or equivalent. At that point, you're overgearing Coil since you don't even *need* any gear from there any more. Try talking about the use of the ability with, you know, numbers for people that *aren't* running around in full absolute BiS. I manage 1.2k on my WAR (relic + 3/5 AF2 and 70/90 jewelry mix), which is what I'm going off of.
Also, you're not comparing IB against a single healer's contributions. You should be comparing it against both healers. As soon as you get into 8 man, the value of Inner Beast *plummets* because of this. 2 healers in gear equal to yours are going to manage more than enough to render your IB heal less efficient than Wrath stacks.
As to the whole "burst v. average" thing, I look at both. Inner Beast is only useful in the sense that you describe it when you've been hit for a substantial amount of damage *without* getting healed immediately before a large spike. Any reasonable decent healer is going to have you either topped off or shielded before that first hit in the first place so Inner Beast is only there for the post-spike recovery, at which point it is *less efficient*, not to mention that, if your healers are healing you regularly like they're supposed to, IB is pretty much *always* at least some portion of wasted overheal (especially when it crit/berserks/etc.). Average mitigation is important because of healer resources. Healers inevitably run out of mp when they're not outgearing content. If you've got amazing spike survivability but no mean mitigation, your healers are going to bottom out faster.
I know how to play WAR. I also know exactly *why* the whole L2P argument is fundamentally flawed and is really only an excuse that ego-driven elitists use to justify their performance when it's really just healers making up for the inadequacies of the class that they just don't want to admit to.