If you want your backups to run faster and you're only writing to one physical spindle, then you need to play with the "pipe", not more threads.  More threads and "parallel" files will just make things run slower.  You need to experiment with the number of buffers dedicated to the backups, packet size, and, if you can, move to compressed backups.  The less you have to send over the "pipe", the faster things will be.

You may also have to look into the settings for the switches, routers, and NICs themselves.  We've had problems in the past where the backup speeds were slower than a USB 2.0 thumb drive.  The network folks figured out some different settings (I don't know what they were) but I've personally run into the previous horror of having auto-negotiate turned on and someone making the mistake of only using half-duplex.

The target disks also play heavily as a part of the "pipe".  If they're slow spinners or don't have much in the line of cache, things are going to be slow.  If the disks aren't dedicated to the backups, things are going to be slow.