According to someone who's actually done the research on how fast Rainbow Dash can fly. Rainbow Dash can fly 5 times the speed of sound with ease. that is shown via a mach cone that forms around Rainbow... this means she's broken the sound barrier. Her normal top speed with she can reach with great ease is mach 5 speed (3,800 mph) When she creates the sonic rainboom, her speed doubles to mach 10 (76,000 mph).
So yes. She is the fastest flier is all of the world. Proof and you lose. ;D
I bothers me to this day she didn't do a Rainboom that episode. I'm hoping they're saving it for Lightning Dust's revenge/return episode where she can do her own version. She might call it something like the Lightning Pulse since EMP is taken and saying it as words rolls off the tongue like a cube. Made of glue.