Being exactly 10 PSI low is strange. However, boosted applications are hard on fuel pressure sensors. Consider replacing the unit and CLEANING the electrical connections to the sensor. Pay attention to the extension harness as well.
When the vacuum reference line is disconnected and plugged, the pressure should go to 40 PSI unless it has been altered by the tune (unlikely).
After replacement, under boost, the gauge fuel pressure should be above 40 PSI.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.