I have helped several clients with this and often the fastest mechanism is USB drive(s). If you do have multiple USB 2.0 ports on separate busses on the server you can get good very good transfer rates. Write out your backups to a number of files equal to the USB drive count. If you are at the high end I have even had companies buy cheapo direct attached storage and moved files that way. Sneakernet can be WAY faster than the existing network, and it doesn't affect existing operations either.