You obviously know our environment since you upgraded us, I just thought I'd mention that I tested this setting for aux copies and found that it cuts our performance in half.
The best performance tweak for us is defragmenting the LUNs, so I've scheduled those to occur every Monday and Friday.
After upgrading to Simpana 9 and implementing dedupe in August, our aux copy speed started off at 47 - 53 GB/hour. Previously, in Simpana 8 with no DDB, we ranged from 75 - 115 GB/hour.
After defragging the LUNs, performance increased to 104 GB/hr, essentially doubling our speed back to what we obtained before deduplication. The difference in speed means finishing our aux copy in ~12 hours vs. over 24 hours.
I then enabled the lookahead reader registry setting but found it halved our performance to about 50 GB/hr. Disabling the setting brought the aux copy speed back to the 100 GB/hr range.
I would like to test various multiplexing factors in the future (currently at 3).
Our magnetic disk is on the low performing side being a Drobo Pro with (8) 2TB disks in a RAID6 array. We may convert our NetApp FAS2020 to be the target sometime in the future and will retest at that time.