Hi,
The hardware compression is vendor specific ( for tape devices). Backup Exec just sends a signal to do a hardware compression, when hardware compression is selected. Thereafter, it is upto the Tape device to do the hardware compression.
The rate of compression depends upon the vendor specification and it differs from vendor to vendor.you can find the information in their respective manuals.
When data is backed up with compression,the compression level varies for different files.For E.g. the executable files,database files are compressed to a less extent as compared to normal data /text files.So the overall compression for the whole data depends upon what kind of data is being backed up.
You may refer the following document,
http://seer.support.veritas.com/docs/199542.htm
Hope this clarifies.