Netbackup multiplexing questions
You must perform buffers tuning first. It's the number one teak for Netbackup. See http://www.symantec.com/docs/DOC4483 page 122 and forward.
You're observations are right (high level). MPX is configured on the storage unit level not policy level. A MPX setting on a policy are used to further limit the setting from the Storage Unit. This way you can say use 5 streams, but if the streams are from the VIP oracle database limit to 2.
But there is more to it. Doing MPX gives a better tape drive usage, but at the cost of restore time. Data written in a MPX stream is interleaved, so the tape drive need to read more data (both the data for restore but also data in the same mpx stream) compared to a non-mpx restore. Make sure you as well configure a fragment size, do not use infinity. A size of 50GB should do the job.
See technote: How "Maximum Fragment Size" for a Storage Unit affects backups and restores
