The radeonsi driver looks like it's doing pretty well. Notable areas of difference with the r600g driver appear to be:
  • MSAA
  • Hyper-Z
  • OpenGL compliance (2.1 instead of 3.1)
  • HDMI audio

Naturally the page doesn't give an idea of the relative performance, but it doesn't look bad by any stretch of the imagination. I'm considering buying Southern Islands card, so I'm feeling pretty happy about this.