the strange bit is that coolant would not aid in acceleration would it?
I always understood that water injection's purpose was to help prevent pre-ignition. As such it wouldn't actually make power, but allow engine settings that create more power. As far as "work" on the piston, I highly doubt that water turning into steam does so at a faster rate than the speed of fuel combustion, which is what would be required for the water itself to actually increase power.