The answer to this is determined by multiplying the native data rate of the tape drive by the compression ratio of the data being backed up. For example, 2:1 compressible data on an HP Ultrium 460 (30 MB/sec native) should achieve a data rate of 60 MB/sec.
Note that the internal pipeline of the tape drive might limit the ultimate data rate. In the case of the HP Ultrium 460, this is around 90 MB/sec. This is unlikely to be a limiter except in the fastest of systems.