By the way... that is bs that the turbo needs back pressure. A turbo, or any hydraulic motor for that matter is makes it's power from differential pressure. So in other words, if you have 20 psi on the feed side and 10 psi of back pressure, you are only effectively using 10 psi to do work. The lower the back pressure, the faster the response of the turbo.
There comes a point however where you won't really notice any improvement. I have a straight pipe on mine, and it didn't make any real difference on mine. I didn't notice anything but sound when I took my cat off and gave it a home in the garage. The muffler was a whole different story. Muffler helped with performance, but I also lost a bit of braking with tow haul.
To be honest with you when I ordered my truck I was really on the fence about a manual as well. I just couldn't get the hang of that goofy low and first gear. To me, something just wasn't right, either low needed to be lower, or first needed to be higher, and the other gears spread out. I thought about getting that, then dumping a 7 or 8 speed in it. I miss a manual. A lot.