First off: Congratulations on getting your TF back OTR. I was going to ask, just before you started this thread, but the thread tells the story pretty completely.
On the "Thin Yellow Pipe" thread, you said that you have left the exhaust flap control solenoid connected ELECTRICALLY, as I understand it. If so, that bit of the ECU functionality does not explain the flat spot on this thread.
Apparently, the ECU learns an engine set-up, and can take many miles of driving to achieve an optimum setting. Perhaps that explains your comment (from the "Thin Yellow Pipe" thread: "...........sluggish performance between 2000 and 3750 rpm which is still there although not as bad as it was. I have not had any feedback from the forums but will contact K Maps to see if they have any idea where the problem may lay.
K maps undoubtedly know more about the ECU than either of us, so their advice if replacement of the Lamda sensor does not work will be invaluable.