The entire graphics card categories is a mess with AMD. Due to the delay, we're either going to see a high end Polaris architecture as none was available during the fall, or we will see Vega. I can't tell you what the model number will be because it's a mess.
AMD was planning on skipping the high end Polaris altogether, in which Polaris would bench plus or minus a few percent around the current 395x the imac uses. The only benefit would be cheaper (apple will win here) & it will run much, much cooler, but don't expect a bump in performance as nVidia is seeing.
The second option is AMD will produce a high end polaris. AMD loses here due to being about 9 months past nVidia on release. We would see on order to a double in speed. The die shrink that FINFET produced allows for doubling the performance at the same voltage & heat. That means AMD could build a Polaris 395x version & we would likely see 70-100% performance boost. That's what nVidia is seeing. Again, AMD loses, & we in.
The third option is AMD sells apple the Vega chips. Everyone who doesn't want to wait loses. This will clearly crush nVidia as AMD is taking what would be the highest end cards that includes what imacs would use for high end, & apply the second FINFET model tweaks to the system. this would be the equivalent of nvidia releasing the GTX 1165 to 1180 models (yes, they don't exist yet).