Only suggestion I can think of is checking for newer network card drivers. Perhaps the method that Opus copies data with has been sped up in later drivers. Probably not, but I can't think of much else to try if you're already played around with the buffer size.
Does copying similar files from the command prompt give you a similar speed to Opus?
(You could create a button to run an external copy tool, e.g. TeraCopy which might be faster with your particular network/hardware/etc.)