You are going to get 100 different opinions of what works. The general consensus is a 4" turbo down pipe back is all that is needed. It will flow considerably more than what you have currently. Changing your exhaust will not create any more smoke at all. Only adding more fuel through a programmer/tuner/module can do that, unless you have bad injectors...then it will be white smoke.
If you are looking to add mileage and smoke, look into either custom EFI Live tuning through McRatracing.com or look into PPE programmers. If you decide to go with EFI, then you will have unlimited potential with tuning, but it does take a laptop and time to adjust the tuning to your truck. EFI Live is the most used tuning on DD.com, due to its ability to fine tune programming to
your truck, not tuning for
everyone's truck.
If you go with PPE, they have already taken the guess work out of the tuning process, but it will limit you to the tunes available to that programmer. PPE is by far the best canned tunes in the business. PPE's tuners work for daily drivers as well as racers, by proiiding several tuning options on one programmer. They have tunes available from 50 all the way to over 400 crank hp. Several trucks similar to yours with this tuner, a built trans and a lift pump have ran high 12 second or lower 1/4 mile times consistently with the Hot+2ET.
If you need any help with PPE tuners or exhaust systems let me know.
Chad