Andy,
Sorry the platform you need isn't supported. The legacy agent is still available in 11d for exactly this reason, but as you mentioned the performance has lots of room for improvement. Meanwhile, RALUS does support 11i and 11iv2 at speeds that should make you much happier. The Backup Exec 11d RALUS has a greatly expanded platform support list from 10 and we are certainly open to adding platform support to RALUS if there is demand. You may want to visit http://enhancement.symantec.com to register your feedback about the supported RALUS platforms.