I'm not sure if there are any options in aticonfig or cccle for disabling video quality options but it would be worth checking. It would also be useful for someone with a high-end 5xx (same age but more shader power) to see how the recent drivers perform on their GPUs. I suspect that their video performance would be fine -- in which case the simpler shaders on the open source driver might be a better fit for your GPU.
maybe i can check it on my mobility x1600. I know it's far away from being high-end, but it's faster than the x1400.