~0.8MB/sec roughly for a 7200rpm SATA drve at the 4k size (maybe worse) :p
Considering LTO2 tape drives, which can go pretty fast stevecs is right as usual not to even mention LTO3 and 4

Stevecs for a situation like that though, could you do something like backup nesting where you have the dll's or whatever backed up by NT backup into a single file and then you copy/move that file off the server being backed up to the server handling the backups and then back the single file up to tape (or just go straight to tape after the NT Backup) which should improve transfer rates significantly to the point where a couple/few drives would be able to provide the required throughput to keep the tape drive fed?

This would theoretically be done with the dll's being backed up ahead of time of the tape backup obviously.