ddboost for rman tuning recommendations?

We recently purchased a new avamar and data domain unit (DD4200), and are trying to tune our oracle rman backups so that using  they complete in our backup window.  We were able to backup a 1.2T oracle db using 8 channels to our old data domain unit, a DD640, in 1.5 hours.  On the new data domain, this backup process is taking over 5 hours.  We get the same poor performance if we schedule in Avamar 7.1 and have it backup to data domain or if we backup directly to data domain using ddboost 1.3 plugin for oracle.

Trying to eliminate the Avamar, we have tried backing up one tablespace (3 datafiles) using the ddboost plug in for oracle 1.3 (1 Channel) to new data domain, and it takes 52 minutes. When monitoring, we see lots of waits on sbtwrite2.  We can back this same tablespace (3 datafiles) up over NFS to the new data domain in 18 minutes.

Any idea on what parameters we should be looking at to get better performance?