The buffer sizes and non-buffered I/O mode are the only real variables (that Opus itself has control over) so I can't think of anything else worth trying. The file copy loop is pretty simple; Opus reads from one file and writes to the other in a loop (reading and writing on separate threads if it makes sense to for the two devices).
One option is to use a button which copies files via the shell, as that will perform identically to Explorer.