The performance numbers you're seeing are quite good.
Your 3590 and 100BaseT technology provide plenty of capacity,
but you won't see full hardware speeds because you're going
through a database system to do all your I/O: ADSM has to be
involved in management and recording, replete with Activity Log,
Recovery Log, volumehistory, error log, and client-server
overhead which includes file system traversal.  The smaller and
more numerous your files, the greater the overhead.
"It all depends...", and so there are no clear-cut answers.
The best you can do is follow the ADSM tuning guidelines and
optimize your hardware in terms of CPU power and lots of memory
for AIX to cache files and database pages.
File-oriented backup schemes like ADSM are inherently slower
than physical image schemes, but provide far greater flexibility,
which has to be paid for.
       Richard Sims, Boston University OIT