Doesn't 3DMark's system info detection tool gather its info before the 3D tests start? In which case the card would still be in 2D mode and report a speed of 300 MHz.
As for the Performance Analyser, that's based on 3DMark2001 data. Are there enough results in the database for the 5800 Ultra? If not you can't get a sensible result out of it, because it won't be able to recognise which card you have.