I read somewhere, perhaps here, a claim that the decrease in power is only a specification change on paper, not any actual change to the engine output. Supposedly there are new DOT noise requirements for trucks measured at maximum rated power. One way to meet them is to derate the engine on paper (i.e. specify a lower rated power/rpm) at which the noise produced is within spec. In other words, the old version also produced 320 HP at 4000 rpm, and the new one can unofficially produce around 362 at 4750 but isn't specified/rated to do so for legal reasons.
I do not know if there is any truth to this claim, though it struck me as plausible sounding. Horsepower and torque curves for the V10 are not all that easy to come by online (at least with a quick search), much less ones comparing the 2016 to earlier years.
At any rate, I would not hesitate to go with the newer model; the gear spacing on the new transmission, particularly between the highway gears, is much improved (with noticeably narrower gaps).
