They tried do some workarounds because HD4k hardware wasn't designed for OCL (AMD needn't it for a GPGPU via ATI Stream)... OpenCL was released after RV770 thats why this card wasn't fully compatible with this API.
OMG... Are You serious? How can they design a hardware without an access to a final specification? (for ATI Stream their solution works great, why? because was designed for this API) Why NV 8k, 9k and 200 series cards was compatible with OCL? Because under the hood OCL was designed based on CUDA solutions in many places (NV have strong position in Khronos and CUDA was more popular than Stream) not on ATI Stream...nothing to do with specifications
The answer is simple. OpenGL codepath in these games are bugged if You have these strange results. For Portal 2 maybe a WINE (designed for NV hardware in many places) or settings are a problem. Sorry, but if 99% titles works good, You can't judge AMD based on 1% of products. I saw similar story with Heroes of Newerth. Users complained about AMD drivers, but they didn't know that a problem is bugged HoN code... We can find many more examples for bugged OpenGL code in apps, which doesn't work properly on AMD cards (AMD cards now allow to do more issues in OpenGL, but this is cause for maintain compatibility with these bugged products; situation is caused by stupid NV policy about failure to comply with an OGL specification) eg. GNOME Shell.
For me discussion is over...