Well, there have been advances in fuel efficiency with diesel engines. A while back (before he started commuting by train) my dad had an old '98 Golf TDI. Proper old-school dirty diesel, got 45mpg day in day out, but plenty of nasties out the back. He then got a new job and it came with a company BMW 320d. This was a 2012 model, much cleaner Euro 5 standard, in a bigger heavier car, much more power (165 hp vs 90 in the Golf), yet the BMW was averaging over 50 mpg. The two jobs were both on the same site, commuting in the same traffic, driven by the same person.
I doubt that the specific fuel consumption of those new engines is worse than the specific fuel consumption of the lump under a Sprinter. What's more likely is that the Stadlers are suffering due to the comparative weights. I can't imagine that the 755s come in weighing less than an equivalent Sprinter given the increased demands for crash safety, the improved passenger environment, and the electrical gubbins for being a BMU. Comparing it to a car again- think how much heavier a hybrid is than a conventionally powered car.
Edit: originally wrote 745s, but it's the 755s that are the bi-modes!
