This is going to sound silly, but sometimes it helps to simpify the
problem by taking TSM out of the picture.

-The best thing I use to diagnose network problems is FTP.  If you think
the problem is in the network, create a big file, and try to FTP it
the problem is in the network, create a big file, and try to FTP it
between the client and the TSM server disk.  If you can't get good
MB/sec throughput with FTP, you are never going to get BETTER throughput
with TSM (because of DB overhead).  (And compare it to TSM times with
COMPRESSION OFF, not ON.)  And ask your network guy to check the switch
for errors while you are doing it.  There are known problems when the
client and switch disagree about the full/half duplex settings, they
only show up as switch errors (and slow throughput).

If you think the problem is in the server disk, compare a backup
running to disk to a backup of the same large file running direct to
tape.  (And watch IOSTAT for high I/O wait times while you are doing
it.)

Simple, but sometimes helpful.