NiZn batteries in my Canon 580EX II flash. Before trying the NiZn batteries, I checked the performance of standard 2700mah PowerEX NiMh in the 580EX II at full power. With the flash mounted on my 7D and both flash and camera in full manual mode, I photographed the XNTimer on my computer screen. My procedure was to get off a shot as soon as the ready light of the flash came back on. Five shots took 15.7 seconds.
Next, the NiZn batteries were installed. Five shots took 8.1 seconds.
From this and a few other tests, my conclusion is that the recycle time with NiZn batteries is roughly half the recycle time of NiMh batteries.
I intend to further test these NiZn batteries in normal use and will report the experience.
Again, a warning: Be patient and don't fire off many shots in a short time because the flash may overheat.