The site has a couple of bits of info:
Read/Write Performance
x1 = 400/400 MBs
x2 = 550/550 MBs - Single drive OR
800/800 MBs - Two drives (RAID 0)
NOTE: Early model Mac Pros 1,1 and 2,1 have PCIe 1.0 expansion slots. Performance of the Solo will be limited by the PCIe bus to about 250MB/s max.
So it sounds as though in your 1.1 speeds will still be decent at 250 max and will almost double when you move to a 4.1/5.1
at least it will work in both computers!