Presently I use the 8cm (80mm) CDs in an Aiwa XP-Z3C http://www.us.aiwa.com/default.asp?page ... t&prod=379
I have used Imation (8cm, 180MB) or Memorex (8cm, 185MB?), it "struggles" with Memorex (8cm, 210MB).
Not sure about 300MB on an 8cm CD-R/CD-RW, I would think it's pushing it a bit with the grooves being extremely narrow, drives may have trouble reading the contents of these discs.
The 8cm CDs are great for carrying data, if the amount of data is approximately 185MB or 210MB, it's perfect and I usually close the session on the disc. If greater than 210MB, then I will use a 650MB / 700MB capacity CD-R and leave the disc open for additional sessions.