I did a lot of testing of network copy speed in Opus over gigabit ethernet recently, and I could not see any overall speed difference between Opus and Explorer.
There were differences in burst speed (which I think are just differences in how the different buffer sizes affect reported speed of the parts being measured rather than actual speed of the entire chain of operations), and also differences with both programs between how long each test took (some tests were much quicker than others, with the same program, but the high/low/average times were equal for both programs), but overall I could not find a significant difference.
If you set Opus to use a 1MB buffer size, that should give you the same characteristics as the Windows 7 version of Explorer when doing network transfers, although in my testing it made little to no difference. (That may not be the case with all disks, networks, antivirus, etc. of course. Some may treat DOpus.exe differently to Explorer.exe, for example. Some NAS are literally only tested/benchmarked against Explorer and some slight differences in the way Opus and other programs open files may trigger pathological performance issues. It shouldn't matter, though, because both Opus and Explorer are calling very high-level APIs that just take a buffer of data and write it to a file, and the rest is up to the OS and drivers. Assuming default non-buffered I/O settings, newer versions of Opus use separate threads to read and write the data when copying large files from a local drive to a network drive, so almost all of the time Opus spends is in waiting for Windows to read or write the data before it immediately asks Windows to read/write some more data.)
Also remember the factors in this post which can affect copy speed. e.g. Copying lots of small files, Opus will be slower than Explorer by default because Opus copies additional attributes that Explorer doesn't bother with.
And remember it's difficult to measure copy speed due to buffering and differences in how the speed is reported. You need to repeat the tests several times in a row (for each program), with the same files, to account for buffering or interference from other programs that may speed up or slow down each test. I did my tests by looking at the Network tab in Task Manager, and measuring the width of the graph for each file copy. (The high and low points on the graphs varied greatly but the widths of each operation -- i.e. the actual time taken to send the data -- were consistent between both programs.)