The story continues
Swapped with a UK manual ECU today. Car started right up, but still has the same 5800 rpm limit, so I started digging in the wires again. Alot of the possible solutions I saw connected to my issue were mentioning the vehicle speed sensor #1 signal (VSS input on the ECU). The ODO gets the speed info for the mileage from the sensor and sends it to the ECU. The odo itself works fine and I traced the wire to the ECU VSS input - perfectly fine as well.
There is a possibility of a cold joint in the ODO (common issue), which is fixed by splicing two wires behind the ODO - tried that as well. No difference at all, still the same low rev limit. The ECU is confirmed to receive proper VSS signal, so this is not an issue regarding (at least this exact) VSS signal.
However, I have some new observations.
1. This low rev limit is not always present - it depends on how you use the throttle pedal. This was the same even when the car had and auto gearbox and ecu and you were in Neutral/Park and tried to rev the car. If you smash it to the floor it will cut at 5800 rpm. If you are more gentle, say 60-70% throttle, it will rev to ~7000 rpm. The car now behaves exactly the same way.
2. So with the knowledge that this happens only on WOT I tried disconnecting the TPS for a test. And voila - the car revs perfectly fine all the way up to ~7k rpm. But that of course immediately throws a TPS error from the ECU, so is not a real solution. Just a pointer in the right direction maybe?
So to sum it up - now with manual TT6 ECU, ECU receives proper VSS signal, low rev limit is only on WOT, does not matter moving or stationary, disconnecting the TPS resolves the issue, but obviously causes error codes.
@TheTurtleshead, I was busy with the manual ECU stuff and did not try your solution with cutting the NSW wire. But this should not be needed, since the car has a manual ECU now.