Interesting - I also have an oem rail (not to mean there's a common cause, just interesting) Which injectors are you using?Curious what you find. My car STFTs to 10%+ on boost onset as well, but then does trend down gradually.
It's not lack of fuel by the LPFP and all 8 injectors are clean but I do seem to have a heavy bias of MPI.
In case it helps you at all. Here are 3 logs of mine.
Specs: R IS38+RS3LPFP+PR925cc MPI on OEM Rail
no i have a vortex XL on the car now; got the L450 originally when building out then found a great deal on the vortex XL. Had to get new wastegate hardware on the L450 so the vortex xl went on and ive been sitting on it since. Had the L450 on for a bit and it made boost quickly, but it was sold to me with the wastegate issue -unbeknownst to me at the time.I see the L450 for sale, did you already give up on sorting this and go back to stock?