ok, i dont know if this is allowed, but i found that doing this fix i went from 20/30 fps on ultra to 60/90 fps with a low dip now and then to 50.
Intel Core i7 980x Six Core Processor @5ghz 12gb ddr3 corsair, 1tb samsung sp corsair 60gb ssd gaming drive.
HD 5970 toxic 4gb 1000/1500
you will find a download link to a dx9.dll, this dll was used by people like me when playing fallout new vegas, this game used the same gamebro engine and version, and i had the same poor performance for my ATI, all you do is place this DLL in your rift folder and it will be loaded, you should see an instant frame rate boost.
it works for me, i hope if work for you, use at your own risk.
edit to add better links
For Ati and nvidia> D3D9Gen_b3-34970 http://newvegasnexus.com/downloads/file.php?id=34970
use this, it will allow you to make a Dll with a selected card, i use a HD 5970, i asked it to make the dll for that card, i used this dll with the game and got even more performance out of it.