My take is, it shouldn't be too much of a surprise that a modern (smaller displacement) diesel engine design can exceed the output of the 27+ yr inline six. By today's standards, that 1st generation Cummins was pretty lackluster. (A modern Cummins is good for twice that) But by the standards of the day, the old 6BT was way ahead of the competition. At that time, the best GM offering was the 6.2 and 6.5 and Ford was using an International N/A diesel, the IDI 6.9 & IDI 7.3. The 1st gen Cummins was a clearly superior engine in terms of both power and design. What many people may not know is that those early model Cummins were detuned. The transmissions of the day weren't up to the torque these engines could put out, so to help extend their survivability, they were "limited" to 160hp/400lb.ft.
OTOH, the engines offered by GM and Ford were reaching their upper limits and maximum potential. To remain competitive, both manufacturers eventually had to replace their older designs and come up with newer engines that could be further developed. Ford in particular had to introduce several engine designs, going from the old N/A indirect injection diesels thru a series of turbo diesels all known as "Powerstrokes". GM as well, had to replace the old 6.5 with the 6.6 Duramax. Since it's introduction it has had to evolve several times to remain as competitive. The Cummins has not changed very much over the years, save for a new head and the use of different injection pumps, the engine is still the same basic design, with many parts being interchangeable between the various "generations".
Even from the very beginning, the original design offered huge potential, and can be easily modified for more power. It's even possible for a first gen Cummins to match or come close to the output of the latest version, simply by turning a few screws and changing the turbo. Had Chrysler/Cummins wanted to blow the competition way out of the water, it could had released their first engine with quite a lot more horsepower/torque that it would had instantly made the GM 6.2 and Ford IDI 6.9 utterly obsolete. This is why I think the Cummins tops the list. Besides this, it has also become the equivalent of the small block chevy of diesels, in that it has the greatest performance support of any diesel and since, has become the most popular diesel engine to swap.
As for the 3.0 Eco Diesel? I believe it's more inline with the expected output for an engine of this displacement. All diesels have a given amount of potential and I suspect the 3.0 is no different. It may get turned up for more power, and maybe someday it might become a popular engine for performance. Only time will tell.
Ed
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