There are probably slight internal hardware differences to boost mileage but overall I would be willing to be most of it is software. The output of 390hp (70hp/liter) in this form, 400hp in the QX80 and 420hp in the Q70 are all complete jokes.
Nissan's VVEL technology is easily capable of 90-100+ hp/liter. To compare, the lowest output of the VQ37VHR, which also has VVEL, in the Inifiniti Q50 is 328hp which is 89hp/liter and that goes up to 350hp in the Nismo 370z which is almost 95hp/liter. On top of that, there are bolt on kits (modded intake manifold, software, long tube headers/cat delete) that put that engine up to almost 400hp (crank) which is 108hp/liter.
Basically I am saying that if you buy a Titan with this engine and you're aiming for anything less than 500 crank hp, you're aiming way too low. That would put it at only (500/5.556) 90hp/liter. The 390hp, 70hp/liter, rating is just typical Nissan sandbagging to meet EPA mileage requirements.
