limited/fixed HPFP pressure, misfire, drop in power, and inability to reach high revs smoothly

I've come here as a last resort. Heartbroken over months of trial and error.

Recently had the engine replaced, and had put in an Autotech HPFP. She has an HPS CAI and a HKS BOV... I know, it's horrible for the engine, but it came with it when I bought it, and I spent all my resources in maintenance. When it was ready, it threw a "running lean at idle" code. After bringing a Mazda electrics specialist in to diagnose and replace the cold soldered fuel fuse, the code was fixed. I ran into some issues with the car not hitting boost targets, which made me think that there was a vacuum/boost leak, but that was fixed after I had her ECU tuned properly with Stratefied.

The current issue is that there is a SIGNIFICANT drop in power, even with the 16.59psi the turbo is spooling. There's basically no torque steer, and she misfires past 4,000rpm. The misfires were less present after the tune, but they're still there. On my AP, she idles at 400-500. Once a blue moon, after not being driven for a day or two, and brought back after a drive, she needs to be throttle played to start after a few hours leaving her in an ever so slightly inclined driveway.

The max HPFP Actual Pressure she hits is precisely 1669psi each time, and thats at 20% throttle when she hits it, continuing at that psi all the way to 100%. She rapidly jumps from 400-500psi to 1200-1400psi and finishes off at 1669psi in less than a second once I put my foot down. She actually drives smoother and faster when I'm at 40-50% throttle than when I WOT, which leads to backfires, and throws a CEL. I haven't attempted to see if she still throws the code after the tune, but I know she still can't climb the revs properly like she should. She goes up well up to 4000, and if she doesn't backfire, she slows down massively during rev climbing, almost stops at 5,500, and throws a P0300 code, along a IC U2064, which is a non-specific code the ECU is unable to find.

The only thing that makes sense to me is that shes not getting enough fuel and is somehow electronically limited to 1669psi. I've read this %1$s

It's a troubleshooting guide for HPFPs:

"If you are having an odd intermittent issue such as fuel pressure fluctuations that bounce around from 400 to 1,500, or something of that nature, then expect the following:
◦ A sticking spill valve that is not allowing the correct amount of fuel into the chamber. (This is especially true on high-mileage cars and on cars where guys run lots of ethanol.)
◦ A stuck-open pressure relief valve.
◦ A bad HPFP housing check valve exit.
◦ Damage to the HPFP internals themselves.
◦ A broken seal screw O-ring or an improperly torqued seal screw."

I don't understand how the mechanical fuel pump can always hit 1669psi perfectly, but I am having my old fuel pump with the OEM HPFP from the previous engine put back in, because I know that one works for sure, and it would eliminate any pretenses towards the mechanic messing up the Autotech HPFP when he first installed it. The guys at Stratefied were apprehensive towards giving me a specific diagnosis, but the overall conclusion was that it might be the fuel rail sensor.

Either way, she's gonna be done on Monday, and will be given a new O-Ring as well, but compared to how things are going, I really don't think that will fix the problem. I've done as much research as I can but I can't find anyone with my exact problem. From what it could be, it might be the fuel pump itself which is getting swapped, the fuel rail sensor, or even a failed PCM/ECU. Hard to love her right now, it's been months since I've had her back to normal. Y'all are my last hope if the fuel pump swap doesn't work.