Did 16hr of highway at 80mph in the disco 1 4.0 the other weekend. I didnt find power to be a major issue even driving through nebraska and iowa with hills+wind. Door seal noise was more of an issue until I jammed 1/4 tubing into the D2 seals I have in the car. Rpms sat at ~2800-2900 constant cruising. Ive also done 32hr of 80mph in a different disco 1 4.0 with 3 pass and enough gear to choke a horse, but that one had stage 6 weight savings in the form of rust holes through all the floorpans

Now admittedly my setup is more or less stock and I was only hauling 1 pass. But you should not have as much issue as you are on rolling terrain or even holding 80mph. I would ensure adaptives are correctly set and you dont have any leaks in intake/plen. Then I would swap to the bigger orange injectors, that had the largest single effect on sluggishness of my 4.0s

 

Land Rover used too small of injectors?

 

are these the injectors you speak of - Bosch 0280155746 ?

 

They were probably fine when brand new and using high quality gas. But by now your truck has been sucking junk gas for ~30yr and those injectors are likely tired and not spraying as well as they did at one point. The big orange injectors have a vastly superior spray pattern and 30 years less use on them of course. Injectors arent that bad of a job, maybe a 2/5 if oil is a 1/5. Perhaps a 3/5 if you have fat hands or a giant lift. Just make sure you get some anerobic gasket maker to re-seal your intake on reassembly.
The big orange injectors and the adaptive reset improved the performance of the car for me a good bit.
Yep

 

My truck would be at 3,100-3,200 or so at 80 mph.


edit: 80 on the speedo would be 2800-2900

80 on the GPS (waze) is 3100-3200

 

The manual trans definitely makes them “feel” faster. :/
but compared to modern SUVs they just aren’t. Sometimes I wish there was a sixth gear, but 80mph on springs, a solid axle, and 100” wheelbase is kinda dicey as it is. It’d be spookier with a lift to me.