Manfred,
In practice, don't forget that your backup is only as fast as the weakest link in your device chain (from the client to the master). Ie: OS of the client, NIC of the client, network speed the client is connected on, network speed between the client and master, network speed of the master, NIC of the master, OS of the master, device throughput of the master to its tape drives (SCSI, fibre, etc).
If you have a bottleneck in there somewhere, your backup will suffer.
I would probably not expect to get anywhere near maximum theoretical throughput from a client. There are so many variables to account for.
$0.02