CM871 low power and fuel economy

[7lucky]

Hi Guys
I have been reading threads on here for a while and they super helpful.
I have a CM871 with EGR and DPF in a western star in Australia which i have owned for 2 yrs.
Ever since i brought this truck somedays it will perform ok then other days i struggle to get 80km/ph with low power almost like a brake is dragging hold me back (which is not the case).
When i purchased the truck it was set at 600hp/1850lb (4.33 ratio diffs) honestly it went like a 430hp truck but then it might go ok for a day or so then terrible for the next few days but this will change every time you start and stop it.
I have done a egr tune and replaced the dpf as it was completely buggered, replaced engine brake shafts and detents due to excessive wear, new engine wire harness, pretty much changed all the sensors, metering actuators looked fairly new when purchased, swapped out timing actuators with 2nd hand ones and got cummins to reprogram the ECM incase someone tried to mess around with it and has not improved its performance.
I have had countless mechanics and auto sparkys look at it with no success, so with all this i demandated the egr and dpf which improved it a little with new settings (575hp/1950lb).
I then had to pull the engine out and rebuild it due to a liner seal leaking (1.4 million kms) and replaced all injectors, head, fuel pump, CAC, radiator, turbo and actuator, all fuel lines, cleaned up all the earth wires from the battery and swapped out ECM's and still no better.
I then got the truck put on a dyno thinking surely this would isolate the problem but still nothing could be found, except we are only getting 515Hp out of it which that's probably the best it will ever do....
Put manual gauges on the fuel manifold yesterday to double check and getting 230 psi at idle and 280 psi at 2000 rpm ( no pressure drop between pump and rail and -10 inHg inlet restriction ).
The only fault code that come up is 111-1 which is low coolant ( also i loose engine brakes when this comes up), ohmed out the wiring back to the ecm and that's fine- replaced sensor and still throws the code but never brings up check engine light unless i unplug the sensor ???
Fuel economy varies from 1km/L to 1.6 km/L pulling 1 loaded trailer.
0.8 - 1.2 km/L pulling 2 loaded trailers and doesn't change to much when empty it really depends on the day on how it wants to perform .
Everyone is lost on why we cannot get this truck any better and i would appreciated any help.
I will be slow to respond as i am not near my computer much .
Thanks