Ok.. Say you leave the stock FPR on. it is going to be at 39 -41 psi with the line on and around 34ish with the vacuum line off. So now you replace the stock FPR with an adjustable one. What are you going to set the FP to?
The same 39 psi right with the vacuum line on. There is no reason to change it from stock.
The only difference is the adjustable one has the provision to adjust the FPR up and down to whatever you set it to. NOT so that you SHOULD set it to anything other than factory. Once you install it, it works the same as the factory FPR. People use the adjustable FPR to overcome smaller injectors when they approach full duty cycle by increasing the FP, or if they run larger injectors by decreasing the FP but that is a band aid solution IMO. They do help when you tune a car and when you start adding a power adder.
Now, for full disclosure I have a mild setup,- explorer top end, 70mm TB and MAF, headers, U/D, and I have a Kirban FPR- only because it came free with the other parts I bought.