First off it's good to keep up with the late breaking news page:
http://www.symantec.com/docs/TECH145136
There are a couple publicly released EEB's in there that would be good to install.
The one you mention and also the one for disk i/o alerts.
Now to discuss more your issue.
1. Rebasing will help to speed up the duplicaiton to tape once it completes. However if you are duping off to tape right after the backup it is likely that rebasing is not keeping up.
2. I did not see above how much space you are using on your dedupe pool so is it less than 32TB or are you using up all 72TB already. This will definitely effect the speed of your system also.
3. do you have the dedupe pool using the 4TB of internal disk on the appliance also or is it only using the external storage for the dedupe pool? The internal disks are running at 3GB/s were as the external are running at 6GB/s. If you use the internal for your dedupe pool then it will downgrade the performance of the whole dedupe pool to 3GB/s for the disk subsystem.
A good option and the recommended path to use if you are duping off to tape is to use the internal disks for an advanced disk and then use the external storage shelves for dedupe only. Set up your SLP to do the backup to the advanced disk and then do your dupe to tape from there and also your dupe to the dedupe pool from there. Have it expire the image after duplicaiton.
This will give good backup and duplicaiton to tape performance as well as keeping the performance up for the dedupe pool itself.
4. I don't see any examples above of what the rest of your infrastucture is like but the things pointed out in point 3 above should cover quite a bit. It should remove most of your current bottlenecks and help your systems to perform faster. Above this it is likely you would have to start looking at other aspects of your backups to find out were issues may lie.
a. what kind of data are you bcking up?
b. what kind of dedupe rates are your getting?
c. what kind of performance are you actually getting out of your network? Have you done any testing on this to see?
Anyway if you have any further questions please let me know.
Thanks