A few ideas for possible alterations:

I'd like a 0 AP Rage that actually only lasts 1 turn. From the current turn to the start of your next.
This would make the ability more-usable without increasing its overall power level. Then we could focus on the intended trade-off of extra damage for loss of defense.

It's kinda odd the way it works now. You can do more damage, but mostly later since you're losing 2 AP to start doing it. You're often trading now damage for future damage. I think Rage would feel better if it gave an instant boost, more similar to Adrenaline.

Or maybe, make it a stance like in DoS1. You toggle Rage mode and gain a constant smaller buff to damage with smaller defense drops, but it costs nothing and is maintained. Perhaps with a cooldown before you can disable it.