Have you got the latest revision low side pressure sensor? 06E 906 051 K
Cam lobe for the HPFP OK and not worn down? Cam follower OK?
If the above checks out, you will need to data log the following:
Fuel Pressure (LPFP) - Actual
Fuel Pressure - Requested
Fuel Rail (HPFP) - Actual
Fuel Rail - Requested
Fuel Rail - Difference
Lambda Control
LPFP should sit at 4.5 bar at idle and cruise, and ramp up to 5.5 - 6 bar at higher loads and/or rpm.
HPFP should sit at 50 bar at idle and cruise and ramp up to 109 bar at high load/rpm for stock tune, and 120-130 bar for remaps.
If the LPFP keeps up with demand, it's usually the HPFP.
See what the difference between actual and requested is and how hard lambda control is working to fill in the gaps.
That ought to give you some clues as to where the issue is.
How many data PIDs can that Snap on log at once? Might be worth doing all the other suspects too, MAF, throttle, engine rpm, boost, timing retard, intake temp etc etc to give you a fuller picture.