with the numbers you are getting- idk if wmi will help much. If you gave egt gauge(s) you could report number on- we would know if it would help.
The way most folks see an improvement is not just using water but the original term of wmi Water/Methonal Injection is the gains from cooling and the methanal itself. Used in a 50/50 mix the water cools the incoming air so more oxygen per cycle can get into the cylinder allowing more power. For the mpg side this requires less happy foot pedal and usng the smaller power increases to be enough. The meth makes the water do the job more efficiently then the meth when it gets to explosion time in the cylinder amplifies the diesel and helps the power which again- you have to keep your foot out of it to get the mpg benefits.
A slightly lesser version of this is an intercooler which requires only initial investment, but requires no maintenance and no tank to refill all the time. Both is best but most folks don’t do both, just an intercooler which is so efficient that it’s impossible to buy a diesel made after year 2000 without one. Thats the second radiator looking thing infront of the actual radiator.
Banks did a good video some years back on his channel explaining how air density is what is critical no boost pressure.
If you do a descent search here you will find some DIY systems as well as guys who ran the complete professional systems which do an amazing job.
Regardless of which you do- I am to the point of STRONGLY recommending a ball valve in the cab that you open/close each time you start & shut off the truck. The systems both use a 12volt electric solenoid- which at some point fails. And if it fails to open- the truck is still drive able. If it fails open, you can save the engine but it will gravity drain water into your cylinders so now before you start the truck you HAVE TO: disconnect and plug the water line. do an oil change, remove all 8 glow plugs crank engine over by hand a few revolutions, then by key a lot of revolutions. Then reinstall the glow plugs, drive the truck and do another oil change within 100 miles or so. Now you put on the new water solenoid and are good until the next solenoid fails. SO… I just say avoid the nonsense and use a manual valve which always works and is cheaper.
OPTION 2
What would be the biggest improvement for you right now imo is a cruise control.
Every car or truck ever does better if traveling for 1 mile continuous speed to be under cruise control than a foot.
Idk how many db2 cruise control units are available out there. They were used in GM trucks & vans, RVs, and in hummers from 1993-1995. All you need to start with is the box with the cable on it and the bracket that mounts the cable to the db2. The controls which go on the turn signal can easily be bypassed by making it power up with the key in the on position and a toggle switch on the dash. A on-off-on momentary. Up will be set middle causes no action and down is coast/off.
You have to feed the box from that switch, from your brake light switch, and the trans/ speed- I can’t remember top of my head but it needs a signal there too.
Without the cruise control system,
What some people do is get a heavy duty cable like the throttle rpm cable used in hmmwvs. It’s like a second throttle pedal that when on freeway you pull out to match your speed then it holds more steady than your foot. Reports on guys doing that is it helps but not as much as the real cruise control and panic moments are bad because you hit the brakes and it doe not automatically shut off so you have brakes and half throttle applied at same time then they jam the throttle cable in fast as possible. While trying to avoid the tesla that just cut them off- haha. So idk if I can recommend that method- that seems more fitting to someone driving past miles of farmland than someone in a city like you or I am.