You're golden like a shower. (or golden like the girls...golden like the arches....and on and on and on...)
I've got a hand full of these setups going right now. They are mother effing awesome for cloning and veg (depending on length of veg).
I don't believe you're going to get a mist much finer than what you're getting...550gph might be overkill too, I've used 396gph and 268gph successfully. I do also use the same spray heads as you, put together with 1/2" PVC and the same fittings.
You could also probably use less than 22 spray heads and achieve everything you could ever imagine, but I appreciate the overkill. Is that a 22gallon roughneck reservoir you're using? I've had issues with leakage around some lids. Your spray heads, although they are "360" don't spray equally in all directions and rather spray a larger percentage of the water in directions of polar opposites. So if you're getting water around the lid, you can turn the spray heads so that their water streams aren't spraying directly at the lid's inside edge. If you don't know what I mean, stick it in the shower with the lid off and the pump on and watch the spray patterns....but I guess if you don't have leaks, then ya know...don't worry about it.
