One more thought here - In addition to any production benefits, the lack of a central instrument cluster seems like it may exist to solve a design problem. In a typical setup with the speedometer behind the wheel, the angle of vision through the wheel for most is relatively unobstructed and the positioning seems pretty natural. For taller people (like me) though, this is not always true. In most cars with this arrangement, I can only make out the bottom half of the gauge, as the rest is blocked by the wheel. For those of us in this boat, it becomes far less useful.
The height line of the dash is significantly lower in the 3, and perhaps this effectively puts everyone in the "tall person" category, as the angle from eye to wheel would be more acute. Looking at some of the photos earlier in the thread it's possible to get a little bit of a sense of this.
It seems like there are really only a few ways to solve this problem while keeping the speedometer in front of the driver. One is to add a HUD. Another is to take the Prius approach and extend the dash upwards above the wheel. Finally you could put the reading on the wheel itself. Each of these comes with tradeoffs: HUD is more complicated to manufacture, an extended dash alters the design, and having a wheel readout brings it perhaps too near for the vision of some, especially given the potentially altered angles. Of these, HUD would seem to make the most sense.
The floating screen that we've seen seems to be positioned to keep the line of the dash intact from the driver's perspective, and perhaps unobstructed forward vision is the priority that drove the decision to locate the speed reading where it is. The expanded forward field of vision (really vision all around) seems like a distinguishing feature and a selling point for this car. This also means that comparisons with other cars with similar setups, like the Renault earlier, may not be apples-to-apples, as the angles will be off. On the other hand, given that no one has actually driven one yet, the ideas here are just as likely to be complete bunk, ha.