I installed DX8 RC1 this weekend. It is as stable as DX7a , and feels quicker. I have not done any benchmarking with it however. I played Starlancer, Baldurs Gate 2(no visible improvement there), Mobil One Rally, and Tribes with it.(of course Tribes is an OpenGL game so that didn't change).
Thats on a K7 700, Abit KA7-100, Radeon 64M, and 256 Infineon PC133