I don't know how MSI afterburner compares to [url=http://www.techpowerup.com/downloads/SysInfo/GPU-Z/ ]GPU-Z [/url], but this little program isn't just great for temperatures, it also can monitor GPU/memory loading, VDDC current/voltage and clockrates.
You can leave it running in the background and it will record information like min/max/avg values for all of those categories. From this you can determine whether you might be hitting a point at which the game is throttling.
AMD cards like mine and your more advanced one seem to have problems with throttling at higher temperatures. For example if I use OCCT to stress it, it starts off at full speed but then loses about 30% of it's performance after a minute (probably in an effort to keep it from overheating).
My experience with this throttling has been pretty miserable. They are so frustratingly sensitive to temperatures and not just the GPU, but the VRM chips that supply the current. I have slightly undervolted my GPU in order to reduce this, but looking at how much the witcher 3 demands, I think there may be some level of throttling happening.
In any case, this is easy to demonstrate. After a cold-boot load up the game look at the FPS while stationary and wait say 10 minutes (the fans should spool up quite a bit when under full load). Recheck the FPS, if there isn't any change, then it looks like it is something else that may be the problem.
This seems to be a pretty interesting read:
http://arstechnica.co.uk/gaming/2015/05/amd-says-nvidias-gameworks-completely-sabotaged-witcher-3-performance/
It sounds like hairworks was a result of Nvidia investing resources in the game during development:
"We are not asking game developers do anything unethical," said Nvidia's GameWorks' Brian Burke. "GameWorks improves the visual quality of games running on GeForce for our customers. It does not impair performance on competing hardware. GameWorks source code is provided to developers that request it under license, but they can’t redistribute our source code to anyone who does not have a license. Most of the time we optimize games based on binary builds, not source code... I believe it is a resource issue. Nvidia spent a lot of artist and engineering resources to help make Witcher 3 better. I would assume that AMD could have done the same thing because our agreements with developers don’t prevent them from working with other IHVs [independent hardware vendors]" CD Projekt Red doesn't have access to the source code, just as AMD doesn't so they can't really do much to optimize it. It is unlikely performance will get any better.
The best outcome I can see here is that this might encourage AMD to be more invested in the future of the game and to perhaps work with CD Projekt Red to release their equivalent technology (perhaps in one of the paid-for expansions).