I was curious to see and hear what would happen if I did just the long tubes and left the rest of the exhaust system completely stock, here's how it went.
I ordered a set of JBA long tubes, O2 sims plus the O2 extender harnesses from URD. The parts arrived very quickly and in excellent condition. The headers have 2 bungs on each side so make sure you have the correct plugs for the bungs that will not be used, mine were not in the box so I contacted Gadget at URD and he had JBA send me a couple plugs within a few days.
The install is basically just nuts and bolts, no special tricks required. The hardest part for me was finding and disconnecting the electrical plugs for the O2 sims located on the driver side, back of the engine, on top of the trans. You cannot see it so everything is done by feel and it was a PITA!
The old stuff came out without a fight and the new headers dropped right in place perfectly. No issues with the headers.
I bolted everything up tight and held my breath as I started her up. I was surprised by the new tone of my truck. It had a very deep mellow tone that sounded pretty darn good to me, now on to the test drive. I pulled out of my neighborhood with my tires spinning, that was something new! The low end torque was very good and it felt very strong above 3k rpm all the way to redline. The sound was not a whole lot louder from the tailpipe but it was much louder coming from under the hood. I was happy with the test so I decided to continue driving it for a few weeks to see how it would work out. After a few days I had problems with exhaust leaking from the headers to y-pipe connection. I ended up purchasing a set of crush gaskets from the dealer and that took care of the problem. A few days later I developed an exhaust leak at the rear of the y-pipe to factory cat-back, this connection had not been disturbed during the header install so I was concerned that a new leak had developed at this location. The connection bolts where fully tight but the doughnut gasket was blown out and leaking, it started to sound like someone farting thru a kazoo, very embrassing to drive around town, needless to say I babied it everywhere so people did not laugh at my truck.
Instead of messing around any further I went ahead and installed the TRD cat-back along with a new doughnut to the y-pipe. Once again I held my breath as I started her up. Man what a different sound this time. Significantly louder with a nice mellow tone even at idle it was louder but in a good way. Another test drive confirmed that my exhaust leaks were cured but I was very bummed that I had lost a lot of that low end torque that I had gained from the headers. I was surprised that it was actually the cat-back that robbed the low end and not the long tubes themselves. My tears dried up quickly once she got above 3k rpm, it really gets going on the top end and sounds great!
So I guess the moral of my story is that if you do the headers only you will achive some nice gains, I could roast my tires at will, big fun! But in the long run I believe that the headers overwhelm the factory muffler and cause exhaust leaks upstream. No good.
If you decide to do the long tubes you should understand that this is a major change to the personality of your truck. You will lose some low end but you will gain it all back and more above 3k rpm. Headers make a very distictive sound that is different from the factory manifolds and it is louder in the engine compartment than stock. I have been driving it now for about 3 months and I am very happy with the change.
This experience makes me wonder what would happen if someone installed a butterfly valve at the front of the cat-back that could be controlled by the rpm to be partially closed at idle and below 3k rpm and progressively open at 3k rpm and continue to open as the rpm increases to redline. This could help give the low end torque we need and give the top end power we want. Any engineers out there??