It should make a huge difference once you tune the trans for correct gears and tire size. Mine shifted absolutely terribly until I had the correct tire size/gear ratio tuned in
So you also have a 6L transmission in your HMMWV?
This system has adaptive programming, I am having no problems at all with shifting now, the only reason I want to program the TCM now is so I can dump a Corvette tune in and activate manual shifting.
I have a hunch I may be able to dump a tune with the EFILive I have now, (Zero Gravity the ones providing the controller are using H P Tuners) I have only tried to download the tune from the TCM, this is not needed as all anyone really needs to be able to do is upload TO the TCM.
There are two reasons I have not tried this, 1 because it is doing VERY well at this time and 2 because I have an event coming up in Feb. in which I will be towing a trailer a total of 80-100 miles down and back on a FWD only beach for the annual cleanup and there is a small possibility I could "brick" the TCM so I want to go ahead and wait until after that event just in case.
The only issue if you could call it that is when running into a head wind AND running 75MPH it will not shift into 6th which is actually correct as the stock 6.2L does not have enough power considering the configuration and weight of this helmet top, but even at 75MPH in 5th I am only turning 2900 RPM which is the same as I was turning at 60MPH with the TH400 and the transmission is just as happy in 5th as in 6th.
If I "coax" it into 6th in these conditions by lifting the throttle and then getting back into the throttle is simply bogs down until it is forced into 5th again, so it is smarter than I am at that point.
This will be remedied by replacing the 6.2L with the 6.5L Turbo GEP I have sitting on the floor in my shop, (again after that event) that extra 50-60HP will change everything I am sure.
If this was a lighter truck like a 2 or 4 door soft top the 6.2 would push it along just fine, that stupid helmet tops aerodynamic property's are horrible, not to mention with all the gear it weighs 2200 LB more than my other M998 the DuraVee.
EDIT, the only caveat is that when I first was working with this transmission I was turning off the batteries on a regular basis and that wipes out the adaptive programming, I finally figured that out and now leave the battery switch alone, it would take 20-30 miles before it would start to level out and work correctly again. If I can manipulate the tuning this learning curve would no doubt be much shorter.