RasterEyes

Actually, I just found a plugin called "Unbalanced". Looks pretty straight-forward, with a nice GUI for performing the task.

Rebooting my array did the trick.
 
I'm not sure if this might be why my array was clinging to these drives until a reboot, but just before checking out all of the various drives, I'd gone into by computer's BIOS and enabled hot-swapping for all on-board SATA ports so I could add and remove drives without a power cycle each time.

Ah-hah!
 
RECONSTRUCT WRITE!!!
 
That's the secret to getting great performance out of my SSDs. By switching to reconstruct write for generating parity instead of auto (which must have always auto-selected read/modify/write), things are going MUCH faster. Along with upgrading my network to 2.5G, I'm getting nearly 250MB/s, ten times the speed I started with.

No need to dis the cheap Fikwot drives either. Use the right settings, and they work great.

I tried a little trick that got the data back without waiting. I excluded disk5 (a disk that didn't have any data on it yet) from the share, applied that changed, then re-included disk5 again. All of my data, including the for-now-emulated data, is available in the video share, without having to wait for disk4 to finish rebuilding.

Now I see what I was missing! For SMB, the remote share was simply "video". For NFS, I need the whole path "/mnt/user/video". I never would have suspected that.