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
tried it, wait like 20min and it made it, in my case mine put on autozombie at 12.5 the x16 card and go up 24mhz, we can tune up it manually and change the tune on zombie mode??
Well, it was some issues on my GPU on 16x riser with perfect zombie mode finding. Ive discovered that in my case 15 is max , giving me 27.8M. you can set it manualy by --zombie-tune 15