Engine/Transmission (1998.5 - 2002) 4" exhaust is it worth it

[KPfeiffer]

My muffler and resonator need to be replaced on my 1999 24valve Ram2500 diesel. I am told by the garage that if I replace the whole exhaust system from the manifold back with all 4" stock I will get better power, gas mileage and less engine wear. Apparently the stock system which is original that is on there now is not 4" all the way. Replacing muffler and resonator is about $250 and replacing all is $1,100. Thanks, Keith