When I was considering an engine for my boat I was looking at an 6.7 cummins at 480hp or the 8.3 at 500hp. I did go with the 8.3 and the marginally higher horsepower but it was not for the "extra hp" reason. Most of the new diesels (under 8 litres) seem to be high rpm, 3000+ engines with a block that is not sleaved. Additionally the horsepower means very little, what is most important is torque and at what rpm it is delivered. In the case of 6.7 compared to the 8.3 the horsepower difference was only 20 but there is nearly 400 lbs of torque separating the two.
Just another spin, pun intended, on the thread.
Not trying to start on argument, but the 6.7 480hp Cummins is 1,165 ft/lbs peak torque at 2,000 rpms, the 8.3 500hp Cummins is 1,327 ft/lbs peak torque at 1,800 rpms. That is 162 ft/lbs torque difference, not nearly 400 lbs as you claimed. I personally think you made the right decision going to the 8.3 500hp. In most cases, the 8.3 500hp is the better choice between those 2 engines. I guess if you were concerned about the extra weight and size of the 8.3, then the 6.7 would be a better option, but the 8.3 is going to deliver better performance in a downeast boat.