[Den-O]

In the context of the show Cyclone Accel Extreme is supposed to be his ultimate, final, and most powerful form. Shroud created the driver and memories which resulted in Skull, W, and Accel. And she is the one who explicitly says that CAX would be the ultimate form. That Phillip would have to ditch Shotaro to use it. But really that's just Shroud's predictions and intentions. But then again she designed the system, she would know right?

Since it wasn't really ever seen in show and only appeared in a few games it's hard to say. The only confirmed things about it are what she said. The games (Climax Heroes) show that this form can use the Prism Bicker and Engine Blade simultaneously as well. We won't ever know how powerful it would have been but I imagine right on par with CJX.