IB is fine as it is. if you increase the duration and decreased the DR, it wouldn't be as effective since you'd be mitigating less on spike which is the whole purpose of that skill.
Also, a longer duration could result in wasted time if the flow of damage isn't constant (e.g. Titan's rotation).

IB is really good and required good knowledge of the fight to be effective. The only problem is the fact that's it's on GDC... I'm working on Twintania right now and it sucks that I have to pause a little bit in anticipation of Death Sentence...