I'll give that a try at some point...What happens if you test both drives individually but simultaneously? No RAID; run two instances of the benchmark concurrently.
When the 240GB was Mixed with the 256GB in HW RAID 0 under linux I got about 470MB/s.
So Mixing these drives puts both into 3.0GB/s mode under Linux set with LSIUtil.
I've not tested them mixed with OS X DiskUtility SW RAID 0.
If you check the AppleLSIMPT driver( Kext ) it has support for three cards, one is this card 1000,50 one is a LSI Fiber Channel RAID card and the last is a LSI SCSI card.
I think I know what the Apple shipped card is, I think it is this card, it has a battery pack, but for $90 I'm not buying one to find out. It's the only LSI SAS card I can find from a G5.
NOTE TO MODS: Not my auction.

Apple 661-3174 / 630-6989 4 Channel SATA 150 Raid Card For Xserve G5 | eBay
Type Raid Card. Manufac turer: APPLE. Part Number: 661-3174 / 630-6989. Alt Part Number(s) 630-6989. Model Number 661-3174. FOR INTERNATIONAL ORDERS.
www.ebay.com
Anyway the Linux driver doesn't like Apple SW RAID and can't detect the Logical Volume, and the OS X Driver doesn't like the LSI HW RAID and can't mount or even do a /dev/entry for the LSILogical Volume.
I'm sure if @joevt is able to get the LSIUtil working on Mac OS PPC it will be able to detect the drives and change the properties.
This is all so damed cool, I wish I could help @joetv with the LSIUtil, but that's over my head, I can build and beta test tho..
The LSIMPT driver from Leopard will likely work on Tiger too, we know Tiger server has one, so if the one from Leopard doesn't work, that one likely will.