I guess I do not understand why you made the 5 SAN clients SAN if you are
not going to go directly to tape.  How big is the Exchange Server.  2 Hours
sounds like a long time for SAN attached 3590s.

The only possible way to do this may be with SANErgy, but I cannot think of
way to share the disk pool between the SAN and LAN clients right now.

My recommendation for this type of configuration is to use Gigabit in the
TSM Server and the SAN clients that you have.  It is cheaper and does
exactly what you are trying to do.  I guess the machines/disk you have are
just too slow to drive the tape drives to effectively use SAN.