Technical question: /dev/sda1 vs /mnt/sda1
 

Is there not a speed loss (via redirection) to writing (via a network) to /mnt/sda1 rather than /dev/sda1?

Not that I'm advocating writing to /dev/sda1.

/dev/sda1 is the raw hard drive partition. It's not usable for storing data until it is mounted to a folder. In your case, /mnt/sda1 is the mounted folder. There is no way to write files directly into /dev/sda1.
In your case, The speed would probably be more related to the network connection and protocol used for file transfer.

Of course, writing to filesystem may be slower than writing linearly to a block device. If you have 100-megabit network, the question is likely moot because 10MB/s network transfer is probably slower than FS write. If you write big files over gigabit network and it is the primary function of one of the filesystems on the computer, it may be useful to test different FS options - they can differ noticeably in large-file write speeds.

Also, using fallocate befoire writing may be useful.

TRue, my wirelss network is the bottleneck. A different FS is not viable as I am making backup disks that are instantly swappable for my Windows machine. This pretty much rules out fallocate as well. :)

Actually, Linux VFAT driver seems to support fallocate somehow. But "backups over WiFi" means that nobody has any reason to care about speed.

I use Microsoft's SyncToy, so speed is not really an issue as it only sends changes, and at least having a backup box in a different room is a lot better than having the original and backup disks in the same machine.

I in any case carry on with other work while the backup is running. Or go and make lunch.