Well, on our one our T2000s, we are doing several things at once.

many large SAN filesystems are mounted directly on the backup server to write 
to tape.  None of this data has to traverse the network so it is very 
fast...80-100MB/sec on a single LTO3 drive.

At the same time, incoming client backups may be writing to a VTL configured on 
the same server.  Those are typically limited to the network bandwidth of the 
gigE link.  If you have enough fast clients and/or parallel jobs active, you 
should be able to get 80-90MB/sec of incoming client backups writing.

And then you may have one or more jobs thats that are copying data from the VTL 
to real LTO3 tape drives (i.e. Vault duplication jobs).  

With the 8 core 8GB T2000, using 4GB FC HBAs to the VTL/LTO3 we've been able to 
do this all with ease.

i.e.
80MB/sec incoming client backups (network->VTL/LTO3)
100MB/sec local backups to tape (local fs ->VTL/LTO3)
100-200MB/sec VTL vault to 1-2 LTO3 drives (VTL -> LTO3)

This is about as loaded up as we have a T2000 and it maybe hits a load of 4-5 
but still 0% iowait and sustained throughput of all three tasks mentioned 
above.  With 4GB FC, you can probably do more.  The host is not the bottleneck.