There are boost pressure limits in practical terms though. Even heavy duty diesel connecting rods have limits to how much force you can transmit through them. Horsepower is torque at a given RPM.
The problem is how much torque can you safely apply with a motor reasonably weighted for a road going personal vehicle. I love my pickup, but I love light & nimble cars as well (actually, my Titan is quite nimble in a straight line, thanks to a 32V V8 motor :)). The only way to obtain horsepower in lightweight engines is to spin them skyword on the RPM scale.
And due to tightening fuel mileage requirements from gub’ment, V6, I6, and V8 diesels with higher RPM capabilities is pretty much the future of diesel.
However, I really think Diesel tech will eventually be supplanted from passenger cars to heavy trucks, by spark ignition engines in the end. More tuneable parameters (spark advance, fuel events, turbo pressure, etc.), and less rotating mass for same power output, and no emissions filters required for soot & NO2. With the advent of GDI, gasoline engines can be built to be just as efficient as diesels, and be able to pull the same heavy loads. Diesel is already begrudgingly produced by the distillers —— they could easily turn that diesel into gasoline. With engine technology bringing gasoline big gains in efficiency, I really think the diesel’s days are numbered. Just too expensive & too heavy.
Even worse, a diesel always wastes some fuel, unburned. That’s the soot... it just builds and builds in the exhaust system and some sort of filter has to either remove it, or it collects and it eventually clogs the system or catches fire. Even with the current crop of multi-event piezo injectors, soot is still a major problem in the end.
Fewer cylinders per displacement quantity and fewer RPMs equals greater efficiency. A 5 liter 6 cylinder is more efficient than a 5 liter 12 cylinder with equal power outputs and equal RPMs. A ten liter engine operating at 1500RPMs is more efficient than a 5 liter operating at 3000RPMs with equal power outputs.
The bigger motor with less RPMs also lasts longer...assuming equal level of manufacturing, materials, and design quality.