I had three cars and I sold one.
Now I have a reprogrammable DTE-Systems power box just sitting around

I have mixed experiences with those boxes.
With small engines are not great compared to a remap, but with bigger engines are great.

https://www.chiptuning.com/us/jaguar...ercontrol.html

They promise +25HP and +48Nm gain.

Did anyone use a "power box" on the JKR?

 

I would avoid it on modern jags (from about 2000 on) as it will basically ruin the fuelling or the PCM will work around it using the O2 sensors (rendering it useless).

The former causes damage, the latter means it does nothing.

 

Well, just got the official answer: for petrol cars, I need a different box.
My box is for diesel

 

Wrong. The box should be avoided.