Just a guess, but you may be hitting a limit of the card or the slot. I would think that write might be slower than read as always. Check that you have the latest drivers for everything. Or, move the drives to the integrated intel controller, enable write back cache and re-run your benchmarks. You should get better scores if that is what you are looking for. I would not worry about the scores you have unless you need for streaming data faster than this. That is not the achilles heel of spinning disks, random read/writes are.
could be the card, Silicon Image is not known to be awesome for RAID (tough for 0 it really shouldn't matter much)
not the slot though PCI-E allows for 250 MB/s on an x1 slot (thats pci-e 1.0)