I know im bumping an ancient thread here, but i was looking for information on this combo and found the thread. I also noted that the last post said something untrue, and i wanted to correct it for the sake of others who were researching the same combination.
(emphasis is mine)
That is just...
“You may not take a 5-foot step using a form of movement for which you do not have a listed speed.”-Players Handbook Pg.144
(which would mean you can take a 5ft step with forms of movement you DO have a speed listed.)
“Down Speed: A flying creature can fly down at twice its normal flying...