I agree with jess that there may be an issue with the impedance on the injectors between the FP and FS.
Here are my thoughts:
- getting a Haltech will not "fix" anything. You will simply be removing the problem by removing the ECU from the fuel and timing equation. It will make it more difficult to diagnose the problem
- larger injectors will be a hinderance. The stock FS injectors are fine for the turbo cars in the US, so they will certainly be fine with anything you have going on there.
- increasing the fuel pressure may get the engine running a bit richer but its a brutal way of doing it. Not recommended
If you do not need the car in a hurry, it may well be worth getting it home and trying to sort out the FS-DE ECU to work on the FP-DE loom. You will need some help from the people on here but they have done this before. It will also need some patience and perseverence on your part.
Hope it all works out