Per the Man..Dale @Tymar:
There is no defuel on the Super Duty trucks regardless of boost amount. There is an overboost code that reduces the cycle time of the IPR. The result is a slower gain in acceleration, not a reduction of fuel. The way to see if you are setting this would be to scan for the code. This is off the top of my head and it may be wrong, but I think it is a P0236 that is the code. You can set them on 94-97's, but it is a higher differential than the 99-03's. I run a MAP Pressure Relief Valve on my 95.
However, I've seen various configurations of MAP sensor alterations and I would warn that most are ill conceived. The majority of the pressure relief set ups I've seen blow off actual boost and do not isolate excess boost to the MAP side only, leaving the manifold pressure able to rise. The electric modifications don't allow MDFR (Mass Desired Fuel Rate) on the low end early enough since they mitigate the overall boost by a percentage