Mmhh... more questions:

Are you saying that the engine IS running leaner than lambda = 1 but because the STFT remain between -1 and +1 LTFT won't change?

Is it possible to tune this way a 98 PCM (like mine) using LS1Edit or is it possible only for newer PCM's and a more powerful software?

Talking to a local car shop that is used to tune European cars they explained that they usually change the lambda target for different areas of the VE table.
What they do is set lambda lower that 1 for fuel economy in low load areas, and they set it up to an AFR of 13.5 for that VE areas used during acceleration (a lower AFR at WOT).
This has nothing to do with PE, it's still in CL.

Many cars over here have a wideband oxigen sensor as stock equipment (Like the Bosch LSU-4): those PCM's don't have a closed loop for lambda = 1 and an open loop with a power enrichment. The lambda value is constantly controlled in every load condition: from idle to WOT.

Can you change the AFR target using the GM narrow band sensors?