> Also using a PCI card for the disc drives in place of the built in SATA
> interface may degrade performance, since the built in SATA is probably
> on a different PCI bus to plug in cards.

You could run a benchmark.  Even something simple like dd to /dev/null
might tell you something useful.

Putting one disk of the mirror on each controller could help if
one of the controllers goes south.  Disks are more likely to fail
than controllers, but controllers can and do cause problems.