As far as I can tell, the stab/elevator config is the same on both aircraft, the PES provides SLOW (1/4 speed) stab control in the event of left and center system failure, if center and right systems in the 757 are lost the stab remains where it was when the failure occurred...the major difference between the two aircraft is the 767 has no automatic trim when the autopilots are disconnected, while the 757 in fact has (mach/speed trim system) failure of this system grounds the 757..which would seem to refute the Boeing claim of no mach tuck tendencies...