Greetings... I've just given this a test.
My card is this PowerColor R9 290X 4GB GDDR OC(BF4 Edition): http://www.powercolor.com/Global/products_features.asp?id=488
In case it is relevant, I also have an nVidia GeForce GTX 650 Ti in the same machine and have left it in for testing purposes.
I will be trying it out with that card removed at some point to see if it makes any difference.
The first thing I noticed is some tearing and general sluggish video performance, however it does indeed work.
The second thing I observed is that the card is appearing as an AMD Radeon HD 8xxx (device ID is 0x67b01002)
It seems to me that particular card should be using the 9xxx driver, so I edited both kext files accordingly which failed miserably and caused the system to crash attempting to load the desktop, so I restarted in safe mode (the nVidia card came in handy for that purpose) and re-installed your original kext files and am back on the 8xxx driver.
PowerColor specifically mentions that this card has the Hawaii chip, which unless I am mistaken is a 9xxx series so I find this all rather strange.
The sluggishness and tearing appear on the nVidia card as well as the AMD card, regardless of wether the AMD card is actually in use (I tested with both screens on the nVidia and nothing connected to the AMD) I presume this is something being introduced into the GUI subsystem by the AMD driver.
My next test will be to remove the nVidia card altogether and put the AMD on the primary PCIe slot to see if it's the combo of nVidia and AMD that is causing the performance issues.
It seems like a good starting point, however the noticeable lag and the tearing while resizing windows is enough to make it unusable at this time.
Hopefully this can be resolved.
edit: I am running 10.9.3