I did some thorough testing on my old 5850 CFX system before deciding to upgrade to 7950 CFX. My middle PC is now a 570 SLI system, so for the top system I was between a 670 solution or the 7950 CFX.
From what I saw from the 5850 CFX system, I was quite satisfied with how the driver copes with demanding games, ie crossfire was operation in everything I tested.
I only have the 7950 CFX system for very little, so my opinion is still forming. So far I have played Crysis 3 with reasonable very high settings, so the game can run at 60fps. There's minimal if any microstuttering. I did see some evident microstuttering in Tomb Raider though although it was also very smooth at 60fps.
If there's something specific I can test please ask (as long as it is not mmos )
Dual gpu systems do have their quirks, but during the past 5 years I've been using dual gpu systems, when I needed the power of the second gpu, it was there and paid off. A user must be conscious of what he must expect though when jumping into a dual gpu system.
PS Oh regarding those Gigabytes, be advised that some newer batches of the WF3 are voltage locked. Mine are too. If you are into OV/OC you should check with the retailer. Hell I checked too, but still got the locked ones lol.
Your PSU will be fine.