It should go below 0.0, I routinely pull the volume to -287 db.
The wave form shown in the track is the that on the original clip. Premiere Pro does not modify the original clip, but applies the volume change when you playback, or export. Its a non-destructive edit.
The only way to see the waveform change is to edit the audio clip in Audition--the operations copies the audio file, you make the changes, and the changed audition file is returned to Premiere. When this is done, the audio clip in Preniere will be colored green and its name will include the word "extracted."