[Viturro]

is true, better intake and better exhaust will give you better engine efficiency. better air filter, cold intake, etc. your truck have turbo?
better exhaust after and before the turbo gives it better response and performance, the problem of the diesel if that the turbo gets pretty dirty and the turbo slow down pretty much, if you have water injection you can clean the intake (pretty dirty too) and the exhaust to give more performance to the engine (just because it cleans the engine) and if you have water inj. maybe you can give to the engine more turbo pressure and maybe it consume less (for same power, or the same for more power, depends on driver), as new audi's tdi engines (35 psi i think).
Short exhaust after turbo is better to.