I've done some more testing with disappointing results.
My original post may have been misleading when I said this issue was present when CrossfireX is disabled. I had been disabling it through application profiles (and the newer drivers have a checkbox that can disable it for anything without a pre-defined profile anyway). When Crossfire is disabled ‘properly’ through the actual CrossfireX menu in CCC, this tearing issue is not present.
Again, none of the simple 2D games (Limbo, Hell Yeah!) have a CrossfireX profile so I didn't expect there to be a difference. I tried to record a video of this tearing and much like when trying to take a screenshot, it is not possible to capture the tear (or seemingly lower frame rate above the tear). It is not present in any videos or screenshots I've made.
This might seem like a workaround but it’s obviously not ideal for anything that actually needs CrossfireX (like Max Payne 3 – demanding graphics, yet tearing in video cut-scenes)
Thinking it could be related to clock speeds (the cards throttle for less demanding requirements like 2D or video) I tried manually keeping the voltage and clock speeds high with no change. I also tried swapping the two cards around in the system with no change.
Going off CrossfireX now...
ASUS P7P55D-E PRO
Intel i5 750 2.6Ghz
1600Mhz 4GB G.Skill – (Dual Channel 2x 2GB CL7-8-7-24)
2x MSI Hawk 5770
Philips 9732 TV/Monitor
Windows 8 x64