I am experiencing exactly the same problem with some D3D9 games, including Unreal Tournament 3 Black Edition; although Alien Colonial Marines seems to be running better on level 3 out of level 4 graphic settings for the first time and it was also developed with the Unreal 3 engine. But, Unreal Tournament 3 is definitely a problem, because it was running fine with earlier versions of the crimson driver, I think somewhere before the 17.x.x crimson drivers mine was running perfectly with my Radeon RX 480 and FX 8350 on all the maps that I am experiencing frame rate drops down to 40FPS now; for example, on map Facing Worlds in the Capture the Flag mode.
To name a few other D3D9 games with frame rate drops from certain angles in the game, is Test Drive Unlimited 2 (On Very High Settings, but fine on just High settings) and Skyrim (but not that often that it affects gameplay). My RX 480 can definitely handle these games on Highest settings, since I have tested it with newer intel i5 CPUs with very good single threaded performance. I think for some reason with the Radeon team optimizing for newer games and Windows 10, it made the Radeon Graphics driver more single threaded heavy in a lot of other games, including DirectX9 games, since the DirectX9 API can only send Draw Calls to the GPU from the CPU through a single dedicated CPU core, even though most CPUs since Quad Cores and Hyperthreading are capable of so much more if it could only be utilized on more cores. This is also a problem with early intel i7s, since their single threaded performance was only slightly better than the FX 8350 for example and I can confirm it, because I experience the same problems on an Intel i7 870 (also 8 threaded CPU).
It does not look hopeful to me that AMD will invest in this, because they are focussing a lot on Vulkan and DirectX12, since it aims to solve the single threaded problem that we are experiencing because of these previous Graphics APIs including DirectX11 and lower and OpenGL that can only make single threaded Draw Calls. One problem is that I have experienced much greater performance increase in Vulkan games than in DirectX12, which I do not know if it is just bad DirectX12 programming or if the API is a failure compared to Vulkan. In addition, popular developers seem to be sticking with DirectX11, where some of the games seem to have almost perfected CPU scaling in games such as Call of Duty Infinite Warfare, Battlefield 4 & 1, Dead Island Definitive edition, etc. But, a lot of newer DirectX11 games such as Hunt: The Showdown from Crytek are showing high single-threaded API overhead which is causing low GPU usage and FPS drops. I do not understand why Crytek is still using DirectX11, because they have already integrated Vulkan and DirectX12 support in their Engine more than a year ago and Vulkan is cross-platform compatible.
From my understanding, the GCN architecture of the Radeon Cards favour more Draw Calls, since it is designed to be very good at Asynchronous computing, but it requires games to make very good use of a Multithreaded Capable Graphics API.
This is a problem for us who have great multithreaded CPUs with not to great single threaded performance, thus limiting us to the amount of games that are enjoyable to play ranging from games as old as DirectX9 to games as new as Vulkan, which is frustrating to get consistent 100FPS+ in Wolfenstein II The New Colussus, but FPS drops as low as 40 in Unreal Tournament 3.
Strong single threaded performance from newer Intel CPUs helps increase GPU usage and FPS in some of the problematic games, but not all such as Arma 3 and Crysis 3 and therefore this will be a problem as long as Graphics APIs that do not support Multithreaded Draw Calls are used.
My current system, CPU: FX 8350, GPU: Radeon RX 480 8GB, RAM: 16GB 1866Mhz.