I have the base model 3.7 quad / D300s upgraded to 16gb.
I'm using Windows 8.1 and the latest AMD Catalyst drivers running in CrossfireX, 2560x1600 primary monitor + a 1920x1200 portrait monitor.
Battlefield 4 is very playable. It's surprising how well it performs on High settings, but I'm not complaining.
Tom Clancy's Splinter Cell Blacklist is playable at High settings, but it has annoying hitching issues in the overworld. I feel like it's a driver issue because it runs very smoothly in some pretty graphically intense areas.
Borderlands 2 works like a dream. Maxed out (no PhysX, obviously)
Skyrim works beautifully, completely maxed out with 8x antialiasing.
Mass Effect 3 never goes below 60 FPS but that shouldn't surprise you.
Bioshock Infinite works great at High details, although there is some screen tearing.
In general, turning on Vsync makes games very laggy / borderline unplayable. Some games have worse tearing than others, but this seems to be an AMD problem in general and not a problem with the performance of the cards. Nvidia's Adaptive Vsync is much better.
I'm pleasantly surprised at how well the base model Mac Pro handles the latest games in Windows. I will try to post some concrete FPS numbers on my blog pretty soon.