Are you performing an AUX copy from your primary disk or backing straight up to tape? Are you using any type of tape encryption? You are able to use multiplexing for databases when streaming to tape. The down side is you have to create an aux copy in the same storage policy to copy the data back on to disk before peroforming a restore. Its perty simple and does not take that long to do. I think the benfits of multiplexing to tape to tape out weigh the fact you have to re-hydrate it back to disk before performing the restore.
This is Aux from disk. Primary is dedup. Aux copy is going non-dedup with hardware encryption. I did some testing with 4x multiplexing and I'm seeing writes consistenly over 200G/Hr now. We just got a new MA which I think will help as well, since the current MA wasn't originally designed with CommVault in mind.