From what I've read in other threads, 2000HP seems to be the average heal from IB+Berserk on geared out WARs (I'm not quite there yet myself). On average, Internal Release would increase this by 20% for 400HP. To get the equivalent HP from Mantra, you'd need 8000HP in healing. This may look high on paper, but isn't an unreasonable amount. You'd also get 2 Berserks for every Mantra, but in situations where AoE healing is needed it seems quite possible for Mantra to win out.

Here's another way to look at it:
The effectiveness of Internal Release scales with your damage.
The effectiveness of Mantra scales with your healers' throughput.
The effectiveness of Awareness scales with the mobs' damage.

At low levels I can see IR being the best option. At higher levels the other two begin to look a little more appealing.