Quite simply, my v2 STi has an ESL daughterboard and 255 pump and is remapped to 320bhp, do I require a further remap to be able to fit and run a FPR?

Thanks.

 

It will fine as long as you set the FPR to the pressure being run now

 

myself, about to do a full parallel fuel rail system. do i need to tune if i raise the pressure by just a very tiny bit. from stock 35psi to 36psi?

 

Any fuel pressure change, up or down will have an effect of some kind.
In closed loop lambda control, a small change won't matter, as the ECU is controlling the AFR. In open loop, in other words anything over cruise settings, a pressure increase will richen the mixture. Whether that's a good or bad thing will depend on where you were to start with!
Some FPR's. rising rate for example, will alter the mixture more percentage wise, relative to load.

 

What is the reason for fitting the FPR?

 

your car already has a fuel pressure regulator, we're just talking about replacing it with an aftermarket unit.

it's there to make sure your injectors stay at a constant pressure