There are a number of reasons we suspect this is an ST and not an early mule for the RS. First is the fact that this prototype is barely more aggressive than the Focus ST-Line model revealed recently. What we can see of the grille, side skirts and much of the rear bumper look like they came straight from the ST-Line variant. The main changes are the inclusion of a dual-outlet exhaust and two versions of a slightly bigger rear spoiler. We would expect a substantially more aggressive front fascia and a much larger rear wing on an RS model. And while the ST currently uses a center-exit exhaust, it would be easy for Ford to do something different on this generation. One other detail that makes us lean toward it being an ST is the wheel design, which looks like an evolution of the standard ST wheels currently available.
Besides design details, there are other reasons we suspect this is an ST. In the current generation of Ford Focus, as well as the preceding European-only generation, the standard Focus was launched, followed by the ST not too long after, and finally the RS. We wouldn't expect that to change for this generation. Plus, the ST is a much bigger seller than the RS, so Ford will probably give that car precedence in development.
So when will we see this new ST revealed, you ask? We'd bet pretty soon. When Ford revealed the new Fiesta, it showed it in a variety of trims, as it did with the Focus just recently. That Fiesta reveal was in November 2016, and just three months later, it showed the Fiesta ST hatch at the Geneva show. Applying that timeline to the new Focus, and we could be seeing it as soon as this summer. Though this is a reveal Ford might want to save for a big auto show, so it might not be until the show season picks up again in the fall and winter. But we'd say we'll at least see it by the end of the year.