If you are using thin devices then it does not matter if the meta member count is not a power of two. The maximum members in a meta is 255 so you are still pretty far from that limit :-)  In thin provisioned environments we see performance benefits for some io type by increasing the meta member counts but the peformance benefits seem to taper off above 16 members so I usually don't build thin metas bigger than 16 members unless I need to do it for capacity reasons.  In thick provisioned environments using meta member counts in powers of 2 helped with the layout on the backend and had an impact on performance.
 I need to expand an 8-way metalun to a 12-way this weekend. Is this metalun config acceptable? is the member count to large? I tested the expansion and it worked ok, I'm just concerned about performance. Also, is there a tool to use to migrate this lun to a different size without needing a host reboot ? I have done a virtual lun migration on VMAX but that is to move a lun to a different thin pool.  Cloning the lun is not an option right now since the host would need a reboot.  Thanks! 
VMAX striped meta-lun member limit