Battle Mage Attack Speed

If I'm using a Fast Staff and Booster, is that the speed Cap? Wondering if it would be better to us Dark instead of Yellow.

Depends if the staff is Fast (5) or Fast (4).
If 4, you'll hit the speed cap with just Booster and can use Dark Aura instead. Evil Wings, Poison Mushroom, Elemental Staves fit here. (Did the later ones get their speeds changed?)
If 5, you need Yellow Aura to hit the speed cap, but using Super Body with it won't get you any additional speed. I know Pyogo Mushrooms are Fast (5). Are there anymore?
If Normal (6), you need Yellow Aura with Super Body to hit the speed cap. Normal staves include the Maple Staves, Revolution Staff, Sun Quan Staff, Crimson Arcanon and the Doomsday Staff.