It's probable that all current cards available now will have performance issues at some level driving the 8K-X at full res/90hz. However its also dependent on other factors.. the application/game being used and the rendering resolution you use..such as if you're sub-sampling or super-sampling.
Its always been stated by pimax from the start "Only for those who own a PC with at least GTX1080ti (still testing, may need 1080Ti SLI, or the next generation graphic card e.g. Nvidia Volta), CPU i7, and 16GB" and basically they don't know.. so we don't either.
As the next generation of GPU's are on the way rumor have it... that's where I would be aiming to run an 8K-X minimum and accept the compromises.