[LTO3 blocksize?]

  bs=     speed=
  32k     50482 kps
  128k    50531 kps
  256k    50508 kps 
  512k    50521 kps
  1024k   50512 kps
  2048k   15780 kps
  4096k   15875 kps

I am in a similar situation with Quantum LTO-2 drives.  I am using 32k
blocksize, and my testing indicated that even at 512k the speed fell off
from the max.  I am seeing 23 MB/s write speed at the peak, and it's
pretty consistent and the drive sounds good at that speed.  I realize
this is slower than the 40 MB/s that LTO-2 is supposed to do.  This is
with dd from /dev/zero, and an Adaptec 2940U2W with only the tape drive
on the bus.

I didn't run amtapetype - I just grabbed one.  I've only written 50G max
of 200G allowed, and usually less so I'm not worrying.

On NetBSD and probably *BSD, 'systat vmstat' shows IO speed per device.