But the focusing speed of the X100 has not - for some time - been an issue. The firmware significantly sped it up. The issue I see has always more related to the use of TTL passive AF that requires the camera to focus, then close the shutter, then open it to expose. This produces a pretty egregious shutter lag that is not going to go away with phase-detect AF. It also induces a ton of wear and tear on the lens/shutter assembly. Phase-detect cuts down on the "over-run" that happens with contrast-detect AF - but it is no silver bullet for low-contrast subjects or low light.
A good way to improve the responsiveness of this camera, especially on moving targets, would be to introduce AF that does not "go blind" when the shutter is closed or during continuous shooting. This could be solved by combining external active (IR) and passive AF information (a la Fuji GA645) or just going to active AF (like a Hexar AF). These cameras had finite numbers of focusing steps, and the systems work well for lenses of the DOF of 35mm f/2 (or 23mm f/2 here).