In your knowledge , assuming a working properly Fuel Pump, does Fuel level in Tank influence rails fuel pressure and engine power ?
It seems to me that the engine runs better with full Tank of gas and become a little sluggish when fuel level in tank drops under half - 1/4 Tank.
Is only my impression ?
My car in NOT stock , i have Gt1 k16/24 s with 1.1 Bar ecu program , stock 3.8 FPR NOT 5 Bar.
Maybe a little fuel pressure drop with low fuel level in tank is normal and not influence performance with car in stock configuration , but when tuned and injector Duty cicle is near 100 % this could influeance engine responce although just a little ?

 

sorry can't help definitively since i'm just a driver but my setup is similar to yours and i believe you would benefit greatly from simply dropping in a 5bar.