Gave the renderer another try and strangely I got it (mostly) playable after switching from core to es, but I still ended up taking a 100+fps hit, running it at roughly 60-100, and experiencing a pretty long precache message between bringing up and closing out of the UT desktop or taking screenshots. There's also a strange issue with how the brightness is handled. On older OpenGL renderers and Direct3D I get considerably brighter lighting in-game and typically have to just adjust it up one (Direct3D) or down one (OpenGL) to get pretty close to the same scene. Under XopenGL however, I have to put it considerably higher and I'll get basically the same scene on the map itself, but things like my HUD, weapon, characters, and pickups will still be far darker than they should be.
What parallex mapping? Unreal didn't have support for that since version 0.83 (October 1996), shamefuly enough....
Unless its something new in these drivers.
Direct3D 10 for Unreal Tournament has it, barely noticeable in-game though. You can see it when you put the camera really close to surfaces, particularly at an angle.