To me, Annihilation damage isn't that big of a deal, and +25-36 sounds about right. All it does is mean foes can't stall with healing spells and that damaged foes are more likely to have downtime. It doesn't mean you deal any MORE damage, so foes take the same amount of effort to kill, it just means your combat abilities can cause a meaningful, lasting effect on your foe much like a mage could curse or level drain them.
[SNIP]
At the level you could first get this (~78th) it can be nasty, but nothing a good bit of downtime and dozens of scrolls of Wish can't fix. At the levels it becomes realistic, (~150-216th) this is a laughable upgrade, and downtime, even with this effect, is almost non-existent.