TBI is junk, pre-1996, low power, glorified carb. CPI is the best system, in terms of power, resembles a TPI system, used for a short duration (believe 1992-1995). the new engine has CSFI, i believe, and are in the middle of TBI and CPI. porting will increase power.
TBI(throttle body injection) is like a computerized carb.
CPI(central port injection) is a step above TBI it has a single injector with 6 poppet nozles going to each cylinder.
the newer 96+ trucks have MPFI(multi port fuel injection) still uses a similar setup to the CPI but fires each cylinder by itself. CPI and TBI batch fire (send fuel to all the cylinders at once.
when I rebuilt my engine I ported and polished my intake. It probably will not greatly enhance your hp but it will be a gain. maybe 5hp without any head work, exhaust, ect.