I believe that it was on the Banks web site a few months back that I read about it. Don't recall the exact words, but it was along the lines that the 6.0L doesn't have a wastegate and uses the EGR valve to dump excess pressure. The EGR actually dumps the boost into the exhaust, and with the exhaust effective blocked, the excess pressure can't be evacuated, with the possibly of building up too much head pressure. Things may have changed with the '07's, but up thru '06, there was a problem when boost go too high, the heads would actually lift and stretch the head bolts because of they were either long enough or strong enough to take the extra pressure.
Seems to me I also read something along the same lines in either the Powerstroke or DieselStop websites.
In all actuality, between the way the transmission works in Tow/Haul along with the was the EGR is programmed, the 6.0L effectively have built-in engine braking ... when in Tow/Haul, foot off the throttle, tap the brakes, the truck does downshift, and does do a good job of slowing me down even with my fiver in tow.
Larry