"--raid-disks" and "--raid-devices" and "-n" are all synonymous.
As the reshape progresses, the distance that data needs to be moved increases, so it is possible that the seek times would increase so the throughput would go down.
You could try increasing /sys/block/mdXX/md/stripe_cache_size.
A larger cache would allow it to read more before seeking, then write more before seeking back.
If you see any performance change, do let us know.