Finding your HPFP Pressure is dropping under boost and you have an upgraded HPFP Internals? This could be the culprit! (Misfiring, Stuttering etc)
 
Located on the passenger (right side) of the fuel rail you find this Fuel Pressure Relief Valve, its job is to relief pressure in the rail once it reaches 1850psi, if this fails it could drop your pressure well beyond the safe level! This is a must and I believe this should be done if you're thinking about going above 200kW and/or your car has done more than 150,000kms on the clock.
 
Idle pressure should be around 380-450PSI when the car is fully warm anything under will potentially be issues with the Fuel Pressure Relief Valve at first instance.
 
Wide Open Throttle pressures should be above 1600+PSI if you see anything under 1600PSI you should confirm if you have an upgraded HPFP Internals OR if you do and it's under 1600PSI then please look at replacing your FPRV here