I had a 50pz800u in the past I can confidently tell you that the picture quality improvement you will see with the new ST line will be astonishing. I wouldn't hesitate. The new Panny's are just about on the same level as the Pioneer Elites.
Just to give you an idea my progression was pz800u, Pioneer Kuro (500m), 65VT50
M&K LCR-750s Fronts and Center
M&K M-4T Surrounds