If your tune is commanding 4300 psi, and that is what your truck makes, your truck is working right. If your actual rail pressure is ABOVE the desired, there is a problem. Most likely, it is too much fuel pressure at the input of the CP3. This can be corrected by turning down the lift pump, or re-scaling the FPR current. If there is a reason you want the high pressure input, I'd recommend re-scaling the current. But you shouldn't need the lift pump that high to maintain rail pressure at WOT.
Once your truck is actually going to the rail pressure it commands, then you should work on making it command the amount you want it to. If you want 9,000 psi at idle, put it in the tune! Or have your tuner do it. Perhaps there is something strange with the LBZ computer, but I have no problem commanding (and getting) 11,000+ psi at idle. Makes the engine sound like a Cummins, though.
As for the pressure drop at WOT, there may be a problem there. I'd do what JoshH said.
