The LSI21320-IS card has "IS" = "integrated striping", in other words
mirroring. If I use its firmware utility to break the mirror back into
two disks:

sd0 at scsibus1 target 0 lun 0: <FUJITSU, MAS3735NP, 5B08> disk fixed
sd0: 70007 MB, 27094 cyl, 8 head, 661 sec, 512 bytes/sect x 143374650 sectors
sd0: sync (6.25ns offset 127), 16-bit (320.000MB/s) transfers, tagged queueing
sd1 at scsibus1 target 1 lun 0: <FUJITSU, MAS3735NP, 5B08> disk fixed
sd1: 70007 MB, 27094 cyl, 8 head, 661 sec, 512 bytes/sect x 143374650 sectors
sd1: sync (6.25ns offset 127), 16-bit (320.000MB/s) transfers, tagged queueing

then the simple test of

  dd if=/dev/rsd0d of=/dev/null bs=64k count=1000

gives a rather more respectable speed of 78392344 bytes/sec (about 75MB/s)

Any thoughts on how to spot what linux is doing that we aren't?
(have kgdb set up + chip manual)

Cheers,

Patrick