So, the car runs fine at 4 psi, and not above that? Sounds like your fuel pump is a big wuss. . .
You really need a fuel pressure gauge to be able to watch for the FMU working. At WOT you will be about 42 psi of fuel pressure, and then at 4 psi of boost, you will be at 82 psi of fuel pressure. The stock pump will start to dead head around 90 psi. So, you may just have a "weaker than normal" pump. It is hard to deliver the necessary fuel when you have to do it at 90 psi and you are used to running at right around 40 psi.
Get an inline or replacement pump and a fuel pressure gauge. . .
Otherwise, save yourself further frustration and sell the parts and car and get something that works (your other idea).