All you need to know is Spartan
Changing back to stock is a PITA, unless you aren't lazy like me and don't mind changing pipes back and getting dirty. I will probably take my chances if I ever need work. I don't plan on ever getting under there and replacing everything back to stock. Waaaay toooo lazy.
The 6.4L in stock mode is the most disappointing thing I have ever driven. In fact, I would have probably gotten rid of the truck if I were forced to drive it stock. I'm running the 310 Spartan
tune every day. It converts the worst diesel engine I have ever known to the absolute best performing, best MPG and best accelerating truck I have ever driven, not to mention that it takes all of the poor diesel traits of lazy low end and mid range and turns it into a V-10 type of power band..
Throttle lag which usually lasted about 2 seconds is replaced by the cracking sound of your neck snapping back as soon as you hit the throttle.
Fuel mileage is what you make of it. Set the cruise at 65 and you will see upwards of 20+. Tow 12K and feel like you need to show off, and you will be around 10.
That's up to you. If you are a kid who gets a woody from burnouts, or generally acting like an inbred redneck
, then plan on taking more chances. If you drive like an adult, I see no reliability issues down the road.