2D physics performance on iPhone

I've been looking into making 2D games with Unity. One thing which concerns me is the performance of the physics engine. Since Unity is a 3D game engine I'm going to have to assume it uses a 3D physics engine. Obviously I can turn this 3D engine intoa 2D one by locking off various axis such that objects only move physically in 2D space.

However this worries me, Box2D being a specialized 2D physics engine is optimized for 2D. Would the physics in Unity3D be fast enough to say, duplicate the sorts of physics we see in games like Angry Birds?