Between the 3 different series of the X15 ( efficiency, performance & productivity) if all things were equal (horse power and torque settings) could you expect the same fuel economy? It also appears that the productivity series is a combination of the other two. If that’s the case can the efficiency and power settings be changed easily by the owner? I can only imagine there is a steep price difference between the three?
I think your trying to simplify it too much.
The higher hp tier engines have titanium turbine turbos and different hard parts to handle more heat etc.. and are inherently less fuel efficient for it.
You wanna play bull hauler go with performance if you want fuel mileage and longevity (less expensive parts) go with efficiency
Is the performance the only one that comes with the dual line fuel pump? It's now known that those are MUCH more reliable than the old single line pumps. It's something to consider given that they won't sell you pump parts anymore and you'll have to buy a complete pump for $8k every 400k miles.
What about the differences in the governed rpm? Is it also because of different hard parts or just for fuel mileage?