I know that Neolith starts out as B speed...
and supposedly Stiria becomes b speed when set as leader.
But I haven't heard anyone even discuss the speed of 7* units since then.
We just have so many that are C speed.
And I feel sorry for any D speed 7* units. They're practically useless in a coliseum event. Hell when using large fruits, only coral and ran even got to DO anything! I got hilarious screens of my two 6* badasses killing a team of 7* and 6* enemies before any of them got to act. 28/30 actions left. hehehe.