There may be some third party tracking plugins for FCPX, dunno. Do you have Motion? It's only $50.
The best motion tracking requires planning when you shoot. Putting colored dots in the corners of the iPhone screen (and better lighting) will help tremendously. Tweaking a good motion tracked effect requires a bit of extra work. For instance, a reflection layer can just have a light streak on in it or it can reflect images in the surrounding environment.
If you get Motion, you will want to experiment with the motion tracking behaviors before committing to the shoot. It's relatively easy and the results are fabulous but it's not a no-brainer.
From the Motion Help System:
Motion tracking is a method of recording the movement of an element (a shape or reference point in a movie clip) in the Canvas, then applying that recorded movement data to another element in the Canvas. For example, you can use motion tracking techniques to:
- “Pin” a post-production graphic to the side of a moving bus
- “Track” a blurry circle to a person’s face to preserve an innocent bystander’s anonymity
- Stabilize a ridiculously shaky camera to vastly improve a “found-footage” thriller
- “Replace” a daring stuntman’s head with the mug of a lazy leading actor
There are six tracking behaviors available in Motion. You apply a tracking behavior to an object in your project—typically a movie clip—to record and analyze its movement. The result of this analysis is a track—recorded movement data—that can be applied to any other object in the project, transferring the motion of a source object to a destination object.