I used to have a travan drive. Here is my experience, as long as you are backing up files that are not flagged to be not compressed, then you should be fine. My 4/8 was backing up 5.5 GB of data.
This is the line in flexbackup.conf that specifies what file types to not compress.
$nocompress_types = 'mp3 Z z gz gif zip lha jpeg jpg taz tgz deb rpm bz2';
GL,
JB