This makes sense... there should be nothing stopping this card from achieving sequential reads of 800MB/s assuming the drives are up to it... which the two Adata drives in RAID0 certainly are.
I've emailed them again... hoping they can chime in on these results above.
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What slot are you running the X2 in and what year Mac Pro? While it shouldn't matter, it might be worth trying this card in one of the slots designed for a graphics card. I'm not familiar with the slot configuration in 2008 and earlier but in 2009 onwards, that would be one of the two bottom PCIe slots. I'm just wondering if there's something limiting the throughput on the Intel chipset or Apples implementation of the PCIe bus on the x4 slots.
My 2 Crucial M4's in RAID0 are capable of that 800MB/s read score as well but its not happening. I've personally tried mine in both the top slots (the slots closest to the drive sleds) in my 2009 with no difference.