I have some good news.
I broke up the current external pool which had mixed 2and 3TBdrives and created a few test pools each with the same size disks. I also removed one disk that appeared to be problematic. Performance improved a bit but nothing dramatic - writes about 50MB avg. Then I rebooted into my Yosemite clone with O3X 1.31r and a dramatic improvement! I backed up a 12GB file in 1 minute vs 4.5 minutes for the same file on O3X 1.5.2. Looks like an issue with O3X.