Ok so after about 9 long and tedious hours of reasearch ive figured out how to make my 290x work with my z270 mb and sierra.
its pretty simple. I followed the guide to the T on rampageDev install method with graphics set to only the 290x on the BIOS but after it has been installed and you reboot to start configuring your macOS sierra you have to disable the GPU ( 290x ) in the bios and use only IGFX or IGPU ( intel graphics ). reboot put your HDMI in the IGPU port , get passed the bios , once you see the clover bootloader , select the boot MacOS sierra ( your drive where you installed it ) BUT DONT PRESS ENTER YET. disconnect the HDMI from iGPU put it in the 290x and then press enter, youre going to fly in blind and once it gets into the OS you should be able to get into sierra and finish your install. I reccomend not doing any of the clover or Kext changes for the GPU ( 290x ) and everything should work ! Hope to help someone , a lot of people are having issues with the 290x and the instructions about bypassing the 290x at bios and boot are kind of slim so im trying to add a little.
if you have questions or want me to explain further , contact me.