Well. I can confirm that in PCI-ex 16x slot even with 16x-16x riser speed is up to 28M.
Was a little mess with zombie auto tune on riser , but then set to auto seems normal.
in my case its 15
Can you link the riser you are using? I ordered this one from a local web-shop. _link_
I found this risers from the othe mess of risers and wires. So I dont remember from where Ive got it. But its like on the picture except power cable.