All things being equal, there is zero difference in the riding experience between the old Z and new Speedplay pedals. Cleats are identical, and the parts of the pedal that mate with the cleats are geometrically identical. Float is essentially identical in degrees of rotation available, and feel of the float. The X series float feels different than the Z/Speedplay float, but lubricating the cleats of the latter pedals helps bring the float quality closer to the former.
All things stop being equal once the pedals begin to wear, though, and the new pedal bodies wear more slowly than the old ones. Plastic surfaces that made contact with the cleats have been replaced by changing the metal “bowtie” to a full coverage plate. As this is the case, comparing used Z series to new Speedplay versions, most riders will experience less “toggle” or lateral movement with the new pedals.
No difference using new cleats with old Zero pedals, which I’ve done for over a year now. Just got a pair of the new Zero pedals for a new bike, will see how they do. Hopefully Wahoo has figured out the hiccups with design/manufacturing snafus that were reported by CyclingTips, among others.