looking at the 4"turbo back kit for my 2006 mega cab. 99,000 miles on the original exhaust and the muffler looks like a grenade went off inside it..the rest of the exhaust looks ok. been kicking around just doing a muffler delete and converter delete. any thoughts? any gains in replacing the factory downpipe? truck is bone stock, only issue ive had is the infamous #4 injector line. i use the truck regulary to haul 10,000 pounds of travel trailer, and our jeep on the trailer. thanks

 

When I did my exhaust conversion from factory stock to my 4" back to 6" stacks, I hooked up pressure gauges and this is what I came up with... stock system had about 5-6 psi of back pressure against the turbo then I first dropped the catalytic converter and gained about 2-3 mpg and the back pressure dropped to 2-3 psi. I then kept the flow thru muffler and went to the stacks in the back close to the cab and the pressure dropped again into the 1/2 psi to 1 psi range and the mpg went up another 1-2 mpg.
Back pressure is not your friend when it comes to freeing up the hp available and the free breathing helps the mpg incredibly.
Just what happened on my rig and setup. The EGT's run cooler as well since the hot exhaust gases are more easily dumped from the manifold and turbo. As the engine runs 'easier', it runs cooler. Cooler means you gain some useable hp and torque to put on the ground instead of using it to turn the engine over and pump the exhaust out.

 

I've put the diamond eye system , turbo back with muffler,on two gen 2 trucks and really like it.

 

Did Diamond Eye 4" turbo back last Christmas (pre twins)(fit nicely when I did the twins too). I used a Banks (wanted quiet) muffler so a bit of "fitting" tail pipe back due to slightly different dimensions. Very happy with fit / finish for the $$$. I don't live in the rust belt, so it will last.
As far as performance, with a CIA and programmer, EGT's would still get stupid hot if I stayed in it but the climb slowed (twins eliminated that problem ).
On a stock truck (04.5 +), your not going to see any real performance gains.