Im 99% sure this is more hardware related than software. I just took an 800mb TDK CDR and put it into my combo drive rev b 12" PB, where Toast told me the blank was around 96mb.
I then put exactly the same blank CDR into my eMac which has a Pioneer DVR 107 superdrive in it (Same as G5's have), Toast tells me there it is 799mb.
Both macs running Toast 6.0.7 and OS X 10.3.5