Thanks everybody for the fascinating links (and Will Davis for the history)!
There are a bunch of technical details that ... haven't been covered in the press releases and Erie, PA, newspaper coverage. I trust more details will emerge as the project matures.
The Jensbacher engine seems to have a per-cylinder displacement of about 380 cubic inches, and full speed is 1500 rpm. (Compare the Caterpillar 3500, as used on new power from some minor builders: 264 cu in displacement, 1800 rpm, for 2000hp from the 16-cylinder version.) My impression [note the word: this is the feel I get from various sources, NOT a professional opinion or one based on specific figures) is that such small, high-speed engines are not as fuel-efficient, in raw terms, as the larger "medium speed" engines like the FDL and GEVO: their advantage in "gen set" switchers, etc, is because whole engines can be shut down when less than full power is needed, so the engines are essentially only used at their most efficient power ratings.
Am I badly wrong about this? If not, where does the fuel-efficiency claimed for the new design come from? The only thing that comes to my mind is that they will use some unconventional (in locomotive use) fuel. The engines are advertised (see the GE website Phil Wormald linked to) as gas engines. I don't know about fuel efficiency (even how to define it when the fuels used are so different), but emissiions-reduction, at least, might be easily attainable by switching from diesel oil to natural gas.
One thought one MIGHT have is that using the smaller engine was a way of keeping the locomotive size and weight down, but I doubt this is the primary reason. After all, the GEVO engine is comparable in weight and size to the "English Electric" (Ruston) diesel design that was successfully used in many British locomotives, such as the Class 50 (which had a 16-cylinder version in a 258,000 pound CC).