The following output has been produced on our Dell PowerEdge 2650 (Gentoo Linux 2004.0) within approx. 15 hours.
It seems that there's no way to turn off the internal hw-compression of this particular tape drive... ;o(

root # amtapetype -f /dev/st0
Writing 256 Mbyte   compresseable data:  34 sec
Writing 256 Mbyte uncompresseable data:  104 sec
WARNING: Tape drive has hardware compression enabled
Estimated time to write 2 * 1024 Mbyte: 832 sec = 0 h 13 min
wrote 1102971 32Kb blocks in 3373 files in 22312 seconds (short write)
wrote 1102043 32Kb blocks in 6761 files in 30812 seconds (short write)
define tapetype unknown-tapetype {
    comment "just produced by tapetype prog (hardware compression on)"
    length 34492 mbytes
    filemark 8 kbytes
    speed 1363 kps
}