Maybe it's the usb stick that your using that's the issue and not the pci-e card , the pci-e card seems to be working and that would not have aything to do with the write speeds simce it did record the read speeds . My guess is that the usb stick is at fault and should be tested on another machine that has usb 3.0 so you can verify the results.
Based on some other articles it seems the bottleneck is the PCIe 1x port, even if it is rev2. The port I have the card in is PCIe x8 and have found a USB3.0 card which is PCIe x4. It will be interesting to see if there is a substantial improvement with the 4x card.