I've managed to half the time the backups take by using striped backups, 10 different files, I have 5 mountpoints for the backup server so this figure seemed logical to evenly spread the backups on the disks (also 10 was faster than 5!).
I read somewhere that adding extra nics to the server may reduce the time further, does anyone have any exerience of this or have any other suggestions to reduce backup time for large amounts of data?