4 GByte per hour sounds to me like you are at 10 MBit, not 100 MBit.

To get to 24 GByte per hour we:
 - Made sure there is NO compression - compressing on the client does not
make sense for Exchange agent data.
 - Set the network ports (on the server, client and switch) to 100 FULL.
Anything set to "automatic" could cause the speed to fall.
 - Made sure the network was defined correctly - allow full 100MBit from
client to server.
 - Staggered the backups so that the Exchange servers were the only things
at that time.
 - Made sure we had Exchange V5.5 SP2, and the latest ADSM Agent.
 - We back up to disk volume.

The ADSM server is also on NT - which some people would say is not ideal.
But it handles what we need very well at present.

Cheers, Chris