My objection to Overpower as a cone was that a cone has a hard time picking up scattered mobs when you're trying to establish Enmity. Even if you're using Tomahawk first to pull from range so they're theoretically starting to bunch up in your direction, there's a still a good chance something is going to stray just outside of the cone's effect. A circular AoE is much more reliable when it comes to hitting scattered mobs for initial enmity.

Using the cone as a part 2 or 3 of an AoE combo would be fine in my opinion. Once mobs are in a tidy pile a cone should have no problem hitting them all.