This only works for some SCSI cards - it seems to be a bit hit and miss. I've generally had some success on my SPARC workgroup servers, but other machines give zilch.
And the actual data transfer rate is something else again. But at least you can check that things are being set up at the rated speeds.
The command is:
prtpicl -v | grep sync-speed
and you then have to work out what the devices are.
Some examples. Here's a V240 with an attached 3310 array:
which shows the intrenal drives at Ultra160, and the external array at Ultra320.
Here's a V440 with a pair of 3120 arrays:
all the disks are at Ultra320, and you see an SES target on each array that's come in at 5M.
Here's an old E450:
which has the disks at 40M, the DVD at 20M, and the tape at 10M.