I've seen the same issue. Disabling DDBoost DSP on the Solaris system will work around it:
1. Touch /nsr/debug/disable_dd_boost
2. Restart NetWorker Services.
I believe the issue is related to the fact that the T5150 has a lot of small CPU's, rather than a few large ones, and a given CPU runs out of juice.
http://www.symantec.com/connect/forums/poor-performances-data-domain-dd-boost-ost-device-solaris for background/analysis for some interesting details