Does anyone know if this fix will be backported?

Not sure if this addresses the same problem (please let me know if not), but a fix that I've used for the past couple of years is one suggested in these discussions:

https://gist.github.com/2E0PGS/f63544f8abe69acc5caaa54f56efe52f
http://lwn.net/Articles/572911/

essentially changing the values of vm.dirty_background_bytes and vm.dirty_bytes. File transfers with the USB have been smooth and fast, on Plasma/Kwin, with this fix.

And this post explains how to make such changes permanent:

https://unix.stackexchange.com/a/149140/515980

Maybe this is useful for people with older versions of Plasma.