Most people should pick up on this pretty quickly, but this is for those who don't.
- The new level 90 boost gives you a 280% flyer and gets you up to 90.
- World boss mounts (even cloud serpents) only require 280% flying to learn (you just can't mount the cloud serpents)
So all I'm really here to say is... using the old "level 1 strategy" (make a toon -> run boss -> delete -> repeat until mount drops) is now more viable than ever, since you can just immediately boost up whatever toon you want to 90 and train it immediately.