BIG is an understatement!!
I held off on installing my 4" Diamond Eye until I could finish my EGT gauge install. The 4" exhaust made over a 100* F EGT difference at 70 MPH. I only had a couple days to test my new EGT gauge before the exhaust install, so my readings may not be real accurate. Also, I have a turbomaster put on by the PO to my PO which may make readings different from a 6.5 with stock vacuum wastegate.
Another big difference I noticed was in spool down time after pulling my foot off the go pedal. Previously, at 70 MPH, the tach would show the RPMs dropped like a rock when releasing the go pedal (back pressure slowed the motor down FAST). After the new exhaust and under the same situation, the motor slowed down much slower.
My install went good. Removing the right side fender liner made a big difference accessing the turbo end of the downpipe. Access was so much better that I decided to order new glow plugs, wait a day for them arrive, and replace them while I had the liner out. I have an extended cab and had enough pipe that I could of put it together without the muffler if I had chosen to. The only thing I had to deal with was the welded on rod part of the clamp that was inserted into the stock rubber mounts didn't always set at the height that I wanted them to. I have a hot wrench so I just modified them to suit my wants. To remove the old stuff I used my sawzall to make one cut a couple inches behind the cat and then snaked it out. I also took the time to paint the new pipes with high temp bar-b-que black paint. It may not be neccessary, but that's just me.
The new system is louder than I had anticipated, but not objectionably so. I did immediately feel a difference with my butt dyno. I just filled my fuel tank and did notice a slight increase in MPG. The biggest problem at this point is that I am having a hard time keeping my foot off the floor. Love that power feel!!
Don