I'm not sure the problem arose when adding this drive, up until now I've never needed to balance as the 2 mybooks were bought at the same time last year and then initially synced with the main array fine. Weekly backups go through fine with them too.
I've tried direct copying from the ntfs mounts of each mybook into the element direct mount both with loads of 3-10mb files, 10gb and then 30gb files, all of which give speeds circa 60 Mb/s for the small files, and the 130 for the larger, and that's whilst the balance is still happening. Turning that off increases the speed slightly even more, so I think I can rule out any USB3 buffering issues.
Both mybooks into the element are the same on performance