I'm a gas mechanic and attended a powerstroke 2014 even at a local dealership. The instructor for the class said the same thing about EGR deleting or blocking off can cause blown head gaskets. I believe he is spot on.
The turbo system doesn't utilize a waste gate, so the overboost when you let off the accelerator has to be purged off through the EGR system and back into the exhaust, so the EGR valve acts as a waste gate. Most will say the VGT system will get rid of the boost, but the turbo is spinning very fast under hard acceleration and there is no way it can go from high boost to no boost that quick when the pedal is suddenly let up.
If the EGR system is blocked off or deleted, the boost has no where to go except into the cylinders causing the heads to lift.
Considering the current state of 6.0 performance, the real world street experiences show it's not a problem. Likewise should one want to install a blowoff valve the parts are out there.
Additionally the exhaust pressure would need to be LOWER than the boost PSI for the flow to "reverse" thru the EGR cooler. Most often the exhaust pressure is higher than that of the boost PSI. Consider the fact the turbine drives the compressor which requires the energy to be used on the turbine side not the compressor side. So you need more energy to turn the turbine to create the energy (boost) at the compressor side.
If you understand the VGT operation of the turbine side you may see a better answer to your supposed issue. In short a wastegate or blow off valve isn't a required part to be used on the 6.0L.
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You see, in this world there's two kinds of people my friend....those with loaded guns, and those who dig.
Taking fuel away is how we throttle, how could that cause an overboost?
I understand drive pressure dropping fast enough pressure escapes from the intake side, but I don't see how that could cause an overboost?