Yes sir - headers make a big difference. Did you calculate your mileage for larger tires? 6 to 7% larger tires, show decreased miles on the odo. My computer is usually about 1 mpg low. I have 2004 with DT headers and 3 inch EMS from headers all the way back. Tires are 285/70 R 18 (33.7 inches).Just put the DT headers on my 2005 LC. Jake Smith at A Rusted Development did the labor. ardoffroad.com. I grabbed a new pair of upstream O2 sensors for him as well. I had the classic ticking exhaust and could see the cracks in the stock manifolds when I went down to pick up the truck. Hairline cracks right around the flange but if you held it at the right angle you could see light through the crack.
I'm very impressed with the difference in the truck. I picked it up and immediately hit a road trip with my boy down to the desert. It was only a 2.5 hour trip down but mixed highway driving at 80mph and then 2 lane roads over a pass at 7,500 feet (soldier summit in UT) and then lots of dirt roads in the Swell. The truck has better pick up for sure and feels like it doesn't have to try as hard. Cruising at 80mph also feels better, and I swear it down shifts less often. I also think I picked up some mpgeees, the computer told me I was at 14.8 since the fill up and that was right after I picked it up. I also did some light wheeling down in the desert on that tank... All in all I'm psyched. Didn't pay much more than cost for manifolds because I found the headers for 20% off and I had to get it fixed anyway. Sounds good too, just a slight difference but I can notice it.
Keep in mind I have a part time kit on the truck and have a flowmaster. Would totally recommend headers vs factory manifold.
NOTE: my headers came with air rails which I didn't need on my truck. We just cut them off. But if you do need air rails better make sure you get the headers with rails.