Here's a link that shows a few gaming benchmarks:
http://www.anandtech.com/show/7603/mac-pro-review-late-2013/10
I highly recommend you check out the rest of that review too, it goes into great detail over several different applications.
Now, firstly, you're going to need to install Windows if you want any good gaming performance. Games for OS X have to be specifically written for Dual GPU support, while for Windows it's simply a matter of enabling CrossFire. Dual GPU support isn't always a sure thing either, as shown in Metro and Company of Heroes 2 benchmarks.
The FirePros do outperform most single GPU setups (which is to be expected). It will definitely run your games well. When the FirePros are used for rending and rely on the GPUs (e.g. editing in Final Cut Pro), those cards just sing.
That being said, I cannot state this enough though: If you are purchasing this computer simply for its gaming capabilities, you can find MUCH better value in building your own computer. Mac OS simply isn't designed well for gaming. That, and this is for all intents and purposes, a first generation Apple product. Like the rMBP and the iPad, support will most likely grow exponentially next generation. Once apps start taking advantage of the dual GPUs, I'm sure the Mac Pro will really start to shine. But it will take time.
In the end, it comes down to what you need right now. If you are working with lots of 4K film or apps that utilize the dual GPUs, then yes, the Mac Pro is most likely worth it, and yes it will perform well in gaming on Windows as an added bonus. Otherwise, you might wish to consider the alternatives. Keep in mind, if you still need to purchase peripherals, you will want to leave room in your budget for them (i.e. It would be pointless to spend $4500 on a computer, then having nothing to spend on a monitor, speakers, mouse, etc).
On a side note, the cards are under the AMD brand. The ATI name has been gone since 2010.