ashift 9 may allow some better space efficiency when storing small data blocks, especially if wider RAIDZ is used, where each logical block has to be split between number of drives. But on the other side use of RAIDZ to store small blocks is not good for performance reasons, since each top-level vdev there has IOPS of single drive. So I would say that ashift 9 is OK, but I would not expect dramatic benefits.