Used a Rendez on the back for a few sticky races. Loads of grip, doesn't seem any slower than a Crossmark overall.
Summer: Ignitor front, Crossmark rear.
Sticky: Ignitor front, Rendez rear.
Mud: Medusa front and rear.
Winter: Ignitor front, Highroller rear.
I have been known to run the Rendez on the front for steep courses (Inners british champs etc).
Racebike gets 62a on all above, other bike gets wirebeaded versions; the harder compound and extra weight make for great training.