I am working on an '02 WRX w/ 60k miles.  The car seems too lack power
in mid-range RPM (3000-5000 RPM).  It's also getting a trouble code of
P0171 (bank 1 too lean).
The factory shop manual has a troubleshooting checklist for that code
which includes fuel pressure, MAP sensor, Idle Air Temp (IAT) sensor,
and Coolant Temp (CT) sensor. I checked the fuel pressure with and
without pressure regulator and the pressures are in the proper range
(36 and 41 psi, resectively).   The manifold pressures are within
proper ranges with key on/engine off and at idle.  IAT reading was
about 5 deg above ambient (within spec).  CT was exactly to spec when
the engine is fully warmed-up.
I also checked the following: air filter is clean, spark plugs are
relatively new with no deposits,
fuel filter is new, the front O2 sensor appears free of deposits, The
blow-off valve has been replaced.  I removed the intercooler and
carefully checked hoses and intake tubes for vacuum (or pressure) leaks
and found none.  I read live sensor data while the engine was running
using an OBD2 scanner and all relevent sensors hade readings that made
sense (O2, MAF, IAT, MAP, TPS, etc).
I am all out of ideas on this one.  I would appreciate any suggestion
of where to look next.