By this I assume you mean the D-phenylalanine (isomer)? I personally found it increased the
duration of a noticeable effect* yet no increase of the peak concentration levels let alone for any extended time amount at these levels which seem (concurrently or separately) doubtful. The analgesics properties of D-phenylalanine are
thought to be via a reduction in the enkephalin degradation (enzymatically, not important here) of which predominately effects DOR (delta opioid receptors), whilst morphine more closely mimics the endogenous endorphins and hence has affinity to MOR (μ-opioid receptor). DOR of which are not known for their euphoric effect.
I would presume your slow peak (a 7 -8 hour peak on morphine - no matter what ROA - is extremely long!) would be due to the slow degradation of the morphine. And lasting effect more a psychosomatic response. Sorry mate.
* Which makes sense since I'm on bupe which is - in addition to all the other opioid-receptor activity - is a DOR (δ-opioid) receptor
antagonist so the prevention of this would cause an extended effect.