True the ECU will try to compensate for rich or lean conditions via the O2
readings.  Only problem is in a lower pressure FI there is less time to
compensate for a lean condition during a cycle as compared to a higher
pressure FI.

The pressure is stock at lower RPMs and loads and during greater loads or
higher RPMs where is more fuel is needed, it is available so the engine can
get what it needs in a shorter pulse rather than going lean.  When I was
running an adjustable fuel pressure regulator, to get it to run good at the
upper RPMs I had to turn the pressure up, but the lower pressure was too
high, to fix the lower pressure the upper PSI was too low.  I also found out
that the AFPR would not limit the low pressure under acceleration and dropped
down to 20 PSI creating a lean condition whereas the stock FPR limited the
low to 28 PSI.

       In doing some research for a problem, I ran across articles about the
Digifant that indicate that the system tends to run lean under hard
acceleration and in the upper RPMs.  I might try this in the near future to
see if it helps.

Others on the list have reported in the past that they have turned the fuel
pressure up to @50 PSI and the engine runs better.

Robert