I also have an IS38 full E85 GTI custom tuned by EQT and had nothing but issues with my PR 450LPFP. The top of the pump that pushed into the basket with the O-ring kept on backing out during WOT. I was getting fuel cuts galore. I removed my pump over 10 times trying to troubleshoot the exact reason for the fuel cuts. The issue was finally resolved when I zip tied the shit out of the pump into place in the basket. Several months later, I started having the same issue and said screw it and bought the RS3 Brushless pump. I got re-tuned by EQT and have never had a problem since. (Since I was getting re-tuned for the RS3 pump, I also added MPI)
As PowerDemon said, logs going from 2k-6.5k rpms would be the most telling. Remove traction control and try to limit wheel spin, 3rd was always useless to me, so I always did my logs in 4th gear (DSG). Pay attention to rail pressure, rail pressure set point, STFT, and Cylinder 1 Injector End Angle Primary Pulse. Rail pressure vs. set point should follow one another, STFT should stay around 0 and Cylinder 1 End Angle should stay around 350. If rail pressure isn't following, STFT is rising quickly and End Angle is getting to around 220 at redline, then those are all telling symptoms of LPFP issues.
