Finding the bottleneck and Performance Tuning is a lengthy process where you have to test the abilities of each component in the backup path.
Please download the Performance Tuning Guide: http://www.symantec.com/docs/TECH62317
On a Solaris media server, you can use OS utilities (tar, dd, etc) to test the media server's ability to write to one tape drive at an acceptable rate. LTO3 should be able to write at +- 100MB/sec.
Find something big enough on the media server itself to write to the tape drive. Test with one tape drive and then write to both drives simultaneously to test media server's ability to write to both drives.
Also see the above guide for ways to test media server.
Next, test client's ability to read data from disk.
Look in the above guide for the bpbkar test - the command is slightly different for Windows and Unix clients.
Do this test for each filesystem on the client.
Next test is the network.
Find +- 1GB file on the client and ftp to the media server.
The above tests will show where the bottleneck is. In most cases it's the client's ability to read and generate data stream.
