The Xorg open source driver has always been slower than the proprietary Windows ATI driver, part of the reason Xorg developers keep clamoring for more support from the video card manufacturers. Instead, ATI (and nVIDIA) would rather make a faster proprietary driver for one platform than help the Xorg guys make it work right for all GNU/Linux hardware platforms.
Nevertheless, the hardware acceleration support is there...just slower compared to Windows. Personally, I'll take "a tad slower than Windows" than "no acceleration whatsoever" any day.