what I understand is all you have to do is let out of it for a second it will shift out of 4X4 and into 4X2 and you will gain power to the 2 wheels considering power loss through the drive train.
Why on earth would you let off the accelerator in a 1/4 mile drag race? Have you ever drag raced before? Or drag raced a diesel or a 4x4? As soon as you let off the pedal, you lose all turbo boost, and the auto transmission will upshift to the next gear. Then after the transfer case shifts (which sometimes can take up to 10 seconds) and by chance the front hubs do unlock (which might not even happen unless you back up), you then depress the accelerator pedal, the truck has to downshift, and spool up all over again. Instead of a 16 second quarter mile time, you're now in the 20's. The loss of power through the drivetrain is extremely minimal with 4wd. Test both. Run your truck in the 1/4 mile in 4wd, then again in 2wd, and you'll see the 4wd run will always have a quicker time because of the higher boosted launch. In street form you can't launch these trucks very well in 2wd because it's very difficult to build boost off the line, unless you have some slicks.
with the right altitude and temps I think 13's is not out of the question but yes gears will be a large factor.
Again with his current mods, 13's are absolutely impossible. No single 7.3L Superduty with his mods has ever ever ever even hit 14's, even in perfect altitude and temperature. The best he could ever hope for with his current power mods is 15's, and that would be if he ran stock suspension and stock sized tires. I've got way more mods on my truck and I'm only running 15.4's at 6000 ft. If I'm lucky I could maybe hit 14.9's at sea level and perfect temps.
I don't see in your sig where you own a 7.3L. So where is your info coming from?