The issue is not just the Processor but the Motherboards.

AMD Motherboards are lagging far behind Intel Motherboards. You are generally looking at PCIe2 vs. PCie3, Slow SATA, no M.2 location, etc.

For Iray (and Superfly, for that matter) the 980Ti is still the best bang for the buck. If you can find a 1080, and can get Iray Drivers for it, then it is a slight improvement over the 980Ti but I am not sure it is worth it yet.

Personally I would spend no more than half your Video Card Budget and get one or two 980Ti then hold out for the 1080Ti.