On paper, they are a mismatch at full volume. On the other hand, I'm not sure I could even be in the room with this amp turned up to 900 Watts. I've never even tured the volume knob on this amp past 11:00 o:clock.
If you plan to actually be within a guitar-cord's length of the amp and cabinet, this setup sounds fantastic and has WAY MORE power than you really need. (just my opinion!!)
+1 The 9.0 should provide enough headroom that you never overdrive the amp. This should be a nice pairing. The lows and low mids on the Uber 212 are incredible; big and warm but controlled with lots of low mid punch. This rig will definitely slam. I've only come close to full output once on an outdoor gig with mine using one side of my SM12.0. It was stupid loud and the low end was still solid.