Well, historically OGL was definitely an epic challenge for AMD, and looks like this here doesn't make a difference. Carmak already emphasized on the whole fact, that Megatexture was a difficult task to build on the PC platform, citing numerous API and driver issues even for a simple operations, like moving/fetching pieces of texture data. One of the hurdles was exactly with the behaviour of the device drivers -- what he wanted to actually do, and how the driver interpreted his intentions as a result... often in unexpected ways.