I was wondering how to reduce the number of devices on a RAID-5 without having to buy extra devices for the move... and you just implemented it!
On Linux 18.104.22.168 I performed a test with a RAID-5 of 4 LVM volumes, hosting an ext3 filesystem. After shrinking the filesystem, I reduce the array size with mdadm from the branch devel-3.1 then I reduce the number of devices to 3 and grow the filesystem. It works perfectly.
Thank you and congratulations for all your work.