I can't believe no priests here have tried to mess around with the invisible mount exploit or even posted about them here. This is not much of a guide, but it shows what else you can do with the exploit.
My WoW is set on the lowest of the lowest graphics setting so i'm sorry if they might be hard to see.
This let's your character to go invisible while the mount stays. (also works in shadowform). What you must do is to time your shadowform to go away just as Dispersion is about to go off. Depending on your latency, I usually can get it off at ~.5 seconds left.
These next two are easier if you can time it well too.
For this one, right after you use Dispersion, wait for it to go down to ~1 second. As soon as it does, recast Shadowform and you will fade back. The fade stops when Dispersion is finished. So if you can get a few different shades of shadow.
This one works if you can get the second one to work. This is probably the hardest one to do I think. What you have to do is to remove Shadowform and cast Dispersion as fast as you can.
First guide here so please be gentle. Also have fun while this lasts. Been on the PTR and casting Dispersion dismounts you .