Michael,
The default policies for Netbackup is the following:
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Veritas-NetBackup
Optimization: TFO
Ports: 13720, 13721, 13782, 13785
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This is "TFO only" which won't give you any compression, only the ability to "fill the pipe". Is your Netbackup traffic already compressed and/or encrypted over the wire? If you turn those features off, chances are you could see really good compression after turning on Full optimization for the Veritas Netbackup application profile.
Just something to try...but I wouldn't recommend turning on DRE/LZ if the app is already doing the compression or encryption.
Dan